The 9600 XT will play any current games fantastically, and any upcoming games exceptionally. In other words, the card should last you a good year and a half before you have to upgrade. And don't screw yourself into buying 256MB over 128MB unless it's only a small price difference, then, why not? But as of now, and the near future, there will be NO use for over 128MB memory.
Unlike the past, MB don't mean a thing, now it's all clock speeds. A while back, if you had 32MB, no matter what it was, the game would play fine, now who cares about MB, I have a 128MB FX 5200 and I won't be playing Doom 3 at 1600x1200.