There are a few guides in the ATI section (search for 9800 softmod). The two things your card should have/do to ensure the highest chance of an XT softmod (BIOS flash) are the following:
1) Run at 412/365 (XT clocks)
2) Have an R360 core (XT core)
It doesn't have to have the 2nd one but it should do the first to ensure that you won't get artifacting like crazy in games/benchmarks. To see which core it has, remove the heatsink and wipe the thermal paste off the core. It should say the core version right below the ATI logo.
Since you're switching from nVidia you should run
DriverCleaner to remove the files that nVidia drivers leave behind that screw up ATI's drivers.
Popular overclocking utilities are
RadLinkier/RadClocker and
ATITool. If you use ATITool you should be advised that it's artifact tester can't be trusted 100% as it tends to exadurate core artifacts and minimize memory artifacts. Also, don't leave the artifact tester going for more than an hour or two unless you want your card fried.
The most popular drivers are whatever the latest ATI has put out. Some people use the hand tweaked
Omega drivers instead of the stock ATI ones.