OK, this should be no problem. The 9800 has built in ability to do fast AGP writes, so you need to disable them in the bios or you will get a conflict. Lucky the Asus gives you the ability to turn it off, some more expensive boards don't.
I have a 9800XT and had to trade my 2 month old Gigabyte board in to fix this problem. (I got an Asus P4S800 and lots of change)