Also just to note,
You mention two nice motherboards (I have that Asus one you mention and love it) then mention you want to continue using your 9800pro.
Those motherboards you are looking at will not take an AGP card, only PCI express.
If you are looking at building a new system, you probably should go with PCIexpress. Not so much because it performs better, but more just to keep your future upgrade options open.
There are boards out that that have BOTH an AGP and PCIexpress which you could consider, but personally I wouldn't recommend it. If you are going the Athlon route, you want the nforce4 chipset, IMO.