if u're looking at AGP cards, get the HD3850, its the fastest single AGP out there, and supports DX10. its on par with the 8800GTS (high-end card).
make sure u have a PEntium D or C2D or AMD X2 or something dualcore to go with it, otherwise u'll bottleneck.
but its AGP so i guess it wont bottleneck as much as its PCI-E counterpart if you're using a single-core chip.
^^^^^^^ assuming ur looking for an AGP solution
otherwise, get a 9600GT if u need a new card and dont have much money. or if u can get an 8800gt, thatd be great too
8600 vs 7600 was pretty much the same, perf wise, except one was newer and supported dx10
8800 vs 7800, 8800 was a LOT faster than the 7800.
HD3850 vs X1950P, HD3850 is MILES AWAY from the X1950P
9600 vs 8600, 9600 is a LOT faster, performs similarly to an 8800GT (highend) but cut down on stream processors and other stuff
9800 vs 8800, not much difference, sadly