The new 7800GTX is apparently about as fast as a 6800GT SLI setup. I don't know if they'll release an AGP version, but the PCI-x version is actually in stock at newegg. $599 isn't cheap but you want the best, right?
For benchie comparision to AH, 3dmark2001SE seems to correlate pretty well on the graphics side of things, but at some point AH ends up cpu limited for framerates so going up in vid card speed will get you better image quality but not higher framerates. For your system, above an nvidia 6800 or 6800GT is in my opinion where you'll see almost no FPS increase but may still get better image quality.