I've got an 8800GT, but I wish I waited since prices for them have dropped a bit. Works great on games like COD4/5, Crysis, GRID, and pretty much anything you throw at it. There are cards that out perform it, but its a good all-around-budgety buy. If you've got dough and want the best of the best, the nVidia GTX280 is the best card you can get at the moment. Although the ATi HD4870 is statistically better than the GTX280, the nVidia still outperforms the ATi card (likely driver related)
If you're buying a computer from a major retailer, i.e. Dell, they will stick you hard for a good graphics card. I've seen vendors charge up to $250 for an 8800GT (which normally costs ~$100 nowadays). I suggest you buy a computer that has a good processor and power supply, and buy a graphics card seperately and install it yourself (make sure you've got a good enough power supply, the appropriate PCI-E slots, and space available on your mobo)