The 8800GTS G92 (512MB) was merely a distraction for the 9 range GPUS (I did buy one though
). Personally, I couldn't live with the noise generated by the 9800GT. Even though a 1GB 9800GT could in theory give a higher bandwidth to the 9800GTX (because of the ram), the 9800GTX will overall, consistently outperform the GT in the majority of test (as it has more stream processors- so rendering power).
The GTX is a much more powerful card then the GT, even with the GT having more ram. It goes 9800GT 512MB, 9800GT 1GB then 9800GTX 512MB, the 1GB version is trying to fill the gap. If you want to know which one performs better, then look no further then the 9800GTX. However if you’re on a bit of a tighter budget, maybe consider a 9800GT 512MB.
Personally I would never go for a single slot graphics card with very high clock speeds (such as the 8800GT or 9800GT), because they produce far too much noise.