As I remmebered i think the problem was with the number of pipes though I can't quite be sure of it it's being some time. The 4x series performed better than any of the 5x series except maybe for the very low 4x series I think was 4200. Though The 5x series started with the pci express slots I believe so some people were forced to upgrade it if they upgraded there motherboards.
Either way these days anybody can get a nice 8x series and up for cheap. Though I recommend never to get any of the lower specs of each gpu series.
4800 is better than any of the 5x series, The 6 series I think most is discontinued except for the revamp of the 6200, 7x series I believe they have 7300le,7600,7900 with gs and only 7900 gt.
But all in all 8800gts sell for a average of 90 dollars so upgrades should be done from here and up. Special note 8800gts is better than any 9600 and below. While 9800gt performes a little better and 9800gtx,9800gtx+ would be the next true upgrade from there worth the money.
Of course True power comes from the GTS and GTX 200 series.