I ended up getting ati 9800pro from fry's for a test drive (they don't have the 6600gt agp). Perf. improvement isn't as big as I hope. I'm only running 740x480 (I have a widescreen), and even with high detail but AA off, simple reflection, HL2 is choppy in outdoor scene. Must be my sad P4 1.8Ghz 512M ram. But even so, I thought there'd be a much bigger gain.
I d/l 3dmark2005, ran the default test, and got 2100. Does that sound about right for 9800pro and P4 1.8?
Edit to say: I guess I should wait for a complete system upgrade. oh I'm going to return the 9800 anyway b/c of the fan noise. That little fan is the loudest thing in my system. Eek.