At the risk of yet another cook in your broth, here are my 2c worth:
You have a Dell Dimension 4600. As with any computer, there is a physical (motherboard-based) slot-based limit on how much RAM you can put on the computer, and there is a functional limit determined by the rest of the design and the operating system.
Your computer uses PC2700 (333-MHz) or PC3200 (400-MHz) DDR SDRAM
(non-ECC). Forget the Pin crap. A while back the industry decided they should just name memory, and so all you have to do is look for those two types. Either one is fine, but the PC2700 is cheaper and your machine will not be faster with the faster PC3200.
How much? Well, per your technical specs Support | Dell US , your computer can take a maximum of 4 GB (8 x what it has now). It has four memory slots, so you’d have to put in 1 GB chips in each of the four slots. This would require throwing out whatever is in there now. Your machine probably needs a lot of fine tuning, but as a rule of thumb adding memory is very very easy and very useful in speeding up a machine.
Right now you probably have two slots occupied with 256 memory chips, giving you 512 total, and two free slots. I’d recommend putting 1GB chips in each of the two remaining slots, if that’s your config, and I would not worry in the least that someone will screw up your computer putting it it. A monkey can do it.
Look here to see how easy it is (scroll to see the section on Memory): Support | Dell US
New 1GB PC2700 is about $35/stick and you need two (for technical reasons, we always replace memory on these types of machines in pairs). That’s $70 new; there are lots of sources for used old memory cannibalized from old machines, and I would not hesitate to buy it. Memory either works or it doesn’t.
There are lots more issues. It’s sort of like deciding what to do with a used car that works pretty well but is starting to get annoying. Somewhere around $100 should get you thinking about a replacement machine since a brand new machine can be had for a few hundred bucks.
Best.