hwbot.org has all sorts of graphics cards (even PoS Intel excuses for graphics) in its database, and they'll tell you average overclocks for all different types of cooling: air, H2O, dry ice, N2O, the list goes on. The average overclock for a 9800 GT is 739 MHz core and 1066 MHz memory on air. That's pretty ambitious for a stock cooler, but not unobtainable; users in the enthusiast category have been able to OC up to 799 MHz core and 1375 MHz memory on the stock cooler (not at the same time, of course
). I would clock your card at those average numbers of 739 and 1066 MHz first; if those turn out to be stable under a stress test like Furmark, trying upping the clocks a little at a time. If not, lower the memory clock first as it has less of an effect on performance than the core clock. OC'ing is all about patience; how much time you're willing to spend tweaking to get the ultimate performance.