With a powerful card like the 6800 series (GT, Ultra) I would be worried about bottlenecking (you listed Socket A 3000+), which means you won't get any closer to the potential of the card itself. I myself, had a K7n2 Motherboard and xp2400+ (hence the name) and after I upgraded to AMD64, my FPS made a giant leap (I run Geforce 6600GT AGP), my FPS doubled from 50FPS on Source to over 112.
My brother has a 6800 (not GT) and he runs a Intel P4 478 2.4Ghz 533FSB, and he has to run BF2 on LOW settings because his CPU and motherboard can't take the performance increase. I recommend FIRST before upgrading the GPU to take the step and upgrade your CPU and Motheboard.
Hope this helps.