And just because the other card is better, doesn't mean the current card won't ALSO play certain games you desire (like WoW) and play them very well. For example, is 80 fps vs. 40 fps vital for your gaming habits, when many games are capped at 30 and look fine? For me that kind of difference would be nice, but not vital. A bigger hard disk, 30" screen, quad processors or 8GB RAM would be "better" too, but if what you have does what you need... then it does what you need.
I would guess (no Mac tests are out yet) that Unreal Engine 3 games like Gears of War would be the most likely to rely on the higher card. Current games may need to be set to less detail, but should still play just fine when you've figured out what settings affect spedd the most. And they can still look good too, just not AS good. (Depending what PC you're coming from, EITHER card may feel like an upgrade in speed and quality.)