The game looks nice on my TFT but does play a bit jerky? Why is that? My settings are Medium to low.
It looks like your system is basically prety good (even though the 6800XT is a budget version, it's still basically a good card).
I reckon that you are either a) trying to play at too high a resolution (even though you quote "settings" you didnt state resolution), although probably unlikely, b) you have a problem with drivers or some such issue.
My money is on b) but of course you would also notice issues with other games too. Try downloading a 3dmark benchmark from Futuremark's website (3dmark05 should be more than sufficient) and compare to others with similar setups, to see if you are getting broadly the same numbers. That's tell you if your system is set up correctly. If not, have you installed AGP drivers (ie the chipset driver, not the device driver)?