Well, r_k brings up an important point. If something you really want to use (whether it be software or a web site or whatever) only works on the PC, then that might be the deal breaker. Something definitely to consider.
Alereon, I can’t speak for the OP but I’m guessing that they know how much the iMacs cost. They don’t use a wide variety of software (just chatting, email, surfing, word processing), all which can be done quite nicely on the Mac. If they are interested in a Mac, there’s really no reason not to get one, as long as none of the “deal breakers” are in place (software that they must use that is not available for Mac, etc).
Frankly, (and I’m not trying to single you out, Alereon) I do weary a little bit of the cost being brought up when it comes to Macs. Because that was the first thing I thought of when I decided I wanted to get one. My gosh, how could it not be? Unless you are a millionaire, you’re going to wonder whether or not you can afford it, and whether it will be “worth it” to you.
So I’m assuming that the OP has figured the cost. If they haven’t checked prices yet, they’ll find that an eMac or iMac are really not that expensive. Or perhaps they’ll decide they’d rather get a cheap PC instead. Whatever. (But speaking for myself, I’ve tried a “cheaper” PC, and boy did I regret it. Windows XP was always freaking out on it, blah blah blah. Cheap motherboard, apparently. So I wouldn’t recommend that route to anyone.) Of course, there’s nothing that wrong with getting a PC (if that’s what a person truly wants), but it gets frustrating (speaking from personal experience here) to have a desire for something “different,” only to have everyone tell you why you shouldn’t do it. Not because it’s a bad and horrible thing, but, because, well, you shouldn’t do it, that’s all. (That’s basically what it comes down to with some people, I swear!) 
If the OP wants to switch to a Mac and indicates that their software requirements are not PC-only, and (once again) they want to switch, and they can afford to switch, well I see no compelling reason why they shouldn’t.
Oh, and a word about software: sure, there is less software for that Mac, but it isn’t that terrible. At least not in my opinion (and I started out on the PC). OS X is now doing OK as far as software goes. So I get to choose between a few good FTP programs instead of several. So what. I only want to use one FTP program anyway. So there aren’t as many MP3 players for Mac. So what—iTunes is where it’s at, baby, why would I use anything else? The main apps that I use (Photoshop, Dreamweaver, Word) are all available for Mac. I’m not suffering.
When I “switched,” I certainly got a lot of people saying (with confused dumbfounded tones), “But it costs more!” Yeah, I knew that. I wanted it. I felt it was worth it. I still do. “But it costs more!” Uh huh. Yep. Know that. It’s my money and I know what I want and I am getting it. “But it costs more!”
And on and on and on. Yeesh.