The Humane Society people are usually very helpful and accomodating - they really want you to find a pet you’ll be happy with. So why not just go in and say, “I need a cat with a dog’s personality” and see if they’ve got anything that fits?
(I think rats and ferrets are awesome, but I don’t really know anything about them. It does sound to me like a ferret would work well for you, though - smart, friendly, playful, energetic but okay with apartment life.)
I’ve had pet rats. They are much better than the other rodent options. (I’ve had rabbits, guinea pigs, and hamsters.) They are as smart as cats and much more personable than other rodents. We used to let them run around free in our apartment. One of them would get into bed with me while I was sleeping and try to wake me up to play. It was an annoying but endearing habit. They’re much better than these other rodents who are essentially frightened of you.
Tarantulas, emperor scorpions, and giant Madagascar pillbugs are all easy to take care of.
You need two rats instead of one, because they need social interaction. Females stink less than males. Rats are cheaper overall than dogs and cats, though finding a vet who can treat rodents is a problem (but at least you live in Atlanta).
I have a very dog-like cat, so they definitely exist.
My vote goes for a bunny. I have two whom I love deeply.
They can be litter trained, are affectionate, and yes folks, it’s true - their shit DOESN’T stink! Their pee is another story entirely, however.
Plan to get them neutered or spayed, and be ready to bunny proof your appt. This means put yer plants up, unless you don’t mind them getting nibbled, and hide your power cords. You can get clear aquarium tubing to put around your power cords if you don’t want to hide them. The bunnies will chew, but it gets much better when they get older.
Anyhow - bunny gets my vote. 
Budgies can be tremendously good companions; if you get a young male and give it lots of attention, it will learn to imitate phrases and will sit on your shoulder for hours and hours; we used to have one that could say maybe a hundred words (some of which were organised into intelligible phrases, although the nonsense combinations were more fun*) - you get pooped on a little bit but it isn’t very messy.
*[sup]His name was Oscar and the first thing we taught him was the full nursery rhym ‘hickory dickory dock’, then we taught him to recite our full postal address (in case he escaped and was recaptured by someone else), then he picked up odd phrases like ‘hello mate’, ‘stupid bird’ and ‘ouch’ (which eventually he would always say as a precursor to nipping my ear); as he got older, he started mixing up words in a most comical way - his favourite phrase became ‘Mouse Bird’ (I guess you just had to be there to appreciate the humour) [/sup]
As a pipe dream, you could get a falcon. Just go falconing on the roof of your apartment, and you can have all the pidgeon you can eat! Of course, falconing takes years of training and lots of permits, but…
How about if you built the ultimate Hampster Tube Farm in your 600 ft apartment. You could have tubes going everywhere.
Think about it.
My friend has ferrets, and they STINK. One could easily completely pollute a 600 sq. ft. apartment if you let it. She also has no sense of smell, so she doesn’t mind them at all. How do people with normal senses of smell deal with it? Do you just get used to it, or do you bathe them every day, or is my friend unusually lax with the ferret hygiene (she doesn’t seem to be, but I don’t know. Maybe she’s not as diligent because she can’t smell it)? I used to think I wanted one, but I just can’t get past the smell. That, and my husband hates them now after one of them nipped at him.
Rats, rats, rats, rats, rats. Even though I developed an allergy to mine, they’re still my widdew puddins.
(That wasn’t baby-talk, my nose is all congested 'cause I forgot to take my allergy meds. snork)
My vote is for Cats, Rats or a Bird.
I have three male rats. Very mellow little guys who adore me and love to crawl all over me while I play on my PC. I chose males mainly because they are more mellow, not too adventurous or chew happy. My biddy boys have a large-ish cage on my computer desk with the door always open. They never leave the desk unless they accidently fall off. But I have them trained to the sound of the treat box rattling, so recapture is very, very easy. Get at least two of the same sex. They’re very social critters. And try to get them from a breeder. It’s a better breed of animal, more healthy and most likely to have had any aggression bred out of them. The breeder I go to gives them to me for free. (colors are wrong, so they are useless for breeding for her, but great companion animals for me)
I have four cats, all males. We have a little “piss war” going on in my house, but I love them anyway. Females might be better since they are less likely to spray. Two cats are also a good idea, since they can keep each other company.
I also have an amazon parrot. Great guy! He has a larger than required cage, and the door stays open, so he has more freedom. You can probably get away without clipping the wings on an amazon. They don’t tend to fly too well to begin with. Mine flys like a brick. He has loads of personality and is great company. You’d probably only want one of these because two would just be too damn load. They tend to try to outdo each other in the noise department.
Hope you find the right critter for you.
I’ll second (or third or fourth) ferrets. They are every bit as personable and smart as cats/dogs; like cats, they can be a completely indoor pet; and a well-maintained ferret does not stink. Their personality is more doglike than catlike and they are highly intelligent. They have the energy of a kitten on speed for years. A patient person can teach them tricks. Last, but not least in an apartment complex, they are nearly silent (or at least the half-dozen or so I’ve known have been, but I’ve never read of any ferret owner complaining of noise).
On the other hand, they require more attention than rats, both in maintenance and safety requirements, and are more expensive to keep. Like dogs, they need to be exercised, but a decent sized room is sufficient (potty training them outside the cage will be the most difficult aspect, IMHO).
Tamex, ferret hygiene is fairly simple. One should change their litter and bedding (including hammocks) every 2-3 days. Frequent brushing, especially with a conditioning spray (usually lightly scented), helps. Ears should be cleaned at least monthly. Ferrets should not be shampooed often - this will actually kick their oil glands into overdrive, meaning that for a few days after a bath they’ll be the most pungent (again, clean bedding will lessen the scent). Lastly, a dietary supplement known as ferrotone (probably a trademarked name, but I don’t know the generic) will help keep their coats soft and the scent down. I’ve also seen water additives that will help control the litter box odor, but I’ve never used them.