Well the cards you are comparing are not even in the same group as in price or performance wise. So it really depends on how much you want to spend. As XFX 7800GS AGP costs around £220 and Ati X700 XT versions around £70 its completely different league.
Ati X700 XT has similar performance to Nvidia 6600GT.
I know you said you dont want to change the motherboard. Considering you have A64 processor changing the motherboard will allow you much better options in GFX cards and for future compatibility.
You can buy 1 of those ASrock 939 motherboards , costs around £40 and buy a Ati X1800XT 256 mb version for £200 so total cost to u would be around £240 where as with XFX 7800 GS still costing you around £220/£230 with no future and you should get better performance with Ati x1800XT.
So it truly depends on your budget.
Amd Athlon 64 3500+Venice @ 2700mhz (9 *300 )
DFI Lanparty NF4 SLI-DR
OCZ Gold VX 3200 @ 246 2-2-2-5-1t
Ati x800 GTO2 @ 540 / 580 XT mod
SB Audigy 2
ViewSonic 19" VX924 3ms
Thermaltake Xaser V