The stock NVIDIA GT 120 can play anything out now, just not necessarily at the highest resolution, depending on the game you'll need to compromise on resolution/graphics settings.
If you're not going to be playing very much, just perhaps the odd Blizzard game (Warcraft, and maybe Starcraft 2, Diablo 3 when they are released) you'll be fine with the GT 120, and can always upgrade later if need be, the ATI 4870 is available in an upgrade kit.
The ATI 4870 is a very fast card, should play almost anything except GTA IV and Crysis at 1920*1200 and maximum settings, so that will give you a more significant degree of future-proofing.
There's always the possibility that Apple will come out with an even faster card than the 4870 as an after-purchase upgrade kit in a year or two time, so if your needs are fairly basic, I would stick with the GT 120 for now. It will cost you a bit extra buying the ATI 4870 kit rather than upgrading right away when you order the computer, but if you never need that faster card for a few years, there could well be a better option available by then.