devine,
the best thing to do if you're buying a game boy is to go to Best Buy and ask for help. They'll tell you all you need to know! My mom just bought my 15 yo brother a gameboy for his birthday, and thats what she did. Its the newest one. (whatever that is! I'm not up to date on my hand held systems either!) I think she said it was about $100.
Basically something in color thats just come out is your best bet. If you buy anything other than the newest model, it just isn't worth it because they stop making games for the old systems as soon as the new one comes out. So you only get so much playtime per system, know what I mean? The good part is, usually you can get half your money back by selling it to a video game store like GameDaze once the newer models do come out. Which you can use to buy the new one!
So, its not a totally bad investment, especially considering how much use it'll get.