The first and worst mistake you made was that you got a Mac, no offence.
Like rwp said, if the upgrade is for games, then you may as well get a PC, its just easier. Or a games console.
Apart from that, if you're set on sticking with the Nazis *ahem* Apple, then you'll need to navigate their walled garden to a newer overpriced laptop with their logo on it. I think they'll be sporting the Radeon 5xxx series now am i right?
Pardon my strong feelings towards Apple, somewhere in there is advice!