Forgive me if this is a little muddled...
I believe that the ATI 9800Pro is an 8X AGP card. you mentioned looking at the 7900. I also believe that the 7900 is a PCI-E interface exclusively. The latter will not function in a PCI slot. I have heard of MoBo's that have both, but I have yet to see one (not skeptical, merely reserved).
That being said, if you are sticking with AGP then your choices are limited. Sadly, AGP is nearing its "End of Life".
I believe that the 7800GS may be a good thought... try these:
I hope that this helps.