There’s a nice video card comparison at ExtremeTech that does a good job discussing the relative performance of all the available products except the 7800GTX. The prices are a little dated, but the information is still valid. Currently, I’d say that the best value can be found at the NVidia 6800GT level. I recently jumped off of the ATI bandwagon (more about that in a later upgrade article), but the X850 XT seems to be price sensitive at the moment.
The price of the PCIe Leadtech WinFast PX6800GT dropped $40 in less than seven days. There are even better deals for AGP video cards.