I've been doing some heavy-duty research on video cards myself, recently:
As far as AGP goes, the 9800Pro is solid at $200, the GeForce 6800 (normal) 128MB is $270 and gives you quite better performance, and the 6800 GT 256MB or X800 Pro 256MB are top-of-the-line at around $400.
In regards to PCI-Express, the X700Pro is a good buy at around $170-$200, although the 6600GT regularly outclasses both it and the 9800Pro and is available for the same price; beyond that price range, Geforce cards are really the only viable ones, as ATI still only offers their PCI-E cards at insanely $600+ price points, and they don't offer performance enough to justify the price; you can get a 6800 256MB for $350 and a 6800GT for $500.
Personally, for AGP I'd go for a Radeon 9800Pro, or a Geforce 6800 if I had the extra $70 to play with, because the performance increase is very nearly two-fold. For PCI-Express, the Geforce 6600GT is by far the best value; beyond that, you run into rapid decreases in price-performance ratios.
Hope some of this helps!
Oh, these prices are taken from Newegg.com, and my info is garnered from Anandtech.com, HardOCP.com, and Tomshardware.com.
Cheers!