Considering the stock clocks of a 9800GTX are only 25/63/128 MHz faster than a 8800GTS G92 I am quite sure you can overclock a 8800GTS G92 to 9800GTX speeds no problem.
EDIT: The 9800GTX is almost $100 more than the 8800GTS G92 at most places. I don't know about you but I wouldn't paying an extra ~$100 for a 25/63/128 MHz increase.
Sure... but I already do that for the most part, since I benchmark each game at 2560x1600. And.. as it stands, the 8800GTS 512 (overclocked) outperforms the GTX in almost all of our tests, so I am assuming the 9800 GTX will be similar. However, this ASUS card is highly overclocked, so the 8800 GTX vs. 9800 GTX might be a fairer comparison.