Here are my recommendations: Slowest to Fastest:
Geforce 6800 (vanilla) ~ $165 (generally faster than 6600GT due to better memory bandwidth)
Radeon X800GTO ~ $170
Radeon X800XL ~ $180 (generally faster in HL2 vs 6800GT/GS)
Geforce 6800GS ~ $200 (almost equal to the 6800GT due higher freqs)
Radeon X850XT ~ $370 (not worth it)
Geforce 7800GS ~ $290 (generally faster than X850XT, but barely)
All of these prices are just averages based on newegg's stock of cards. Speed
comparison of cards are just from my personal research, someone else's experience
may be different. Some have mentioned SLI, this is not supported by AGP version
of the cards.
Worthy consideration:
X800XT AIW at Newegg ~ $290 (one of the fastest cards just under the X850XT.)
C Snyder