4870 might be overkill for most games, but crysis not so much, my friend only gets under 30 fps on crysis v high no AA in the final stages of the game. of course Crysis v high is not a good example at all, but if you wanna max out warhead and vanilla crysis, which he plays alot, it could be worth it. so imo its best off the go with a 4830 or 4850, but the option is always there.
btw my friend has 19" wide and 4870/q6600@3.2GHz