PS3-Mac comparison is fair, at the moment at least, Macs don't have a lot of games, neither does the PS3, Macs are overpriced, so are PS3s, Macs are used for lots of multimedia purposes, and what do you know, so are PS3s!
Don't worry about the extra RAM, go with the hard drive space or possibly better graphics, or you could save your money for Leopard which comes out in December, or even better, in stead of spending over a grand, spend $600 (probably a little more Canadian dollars for some reason) and buy a brand new gaming desktop, it might not be the very best gaming machine but it will have better specs than the Mac, you can even (illegally natively, legally through emulation) run OS X on it!