Is it the mobo or video?

Okay, I had an Abit IT7 Max2 V2 mobo and MSI GeForce 5900 running great for the last few years. I started playing Oblivion but had some slowdowns so I decided to upgrade to an MSI GeForce 6600. The only one I could find was an 8x AGP and my mobo only supports 2x/4x. I decided to try it anyhow and it didn't work. No big deal because I was kind of wanting to get another mobo anyhow. I upgraded to an Asus P4S800D-X, which supports 8x. I've gotten everything loaded up but now graphics in Oblivion are 10x worse. I can't get past the intro video because it's moving at about 1 or 2fps. I loaded 3dmark 2006 just for kicks and I scored 915. However, I can actually get older games, such as UT2003 to actually fun fluidly. Most of the tests in 3dmark were running at 2-5fps though. I no longer have my old video card to test the AGP slot with though. I've loaded all of the SIS chipset drivers from the cd and also loaded new ones from the Asus website and the new ones from the SIS website to no available. I've bumped up the AGP aperature to 128MB and ensured the video preference in the BIOS was set to AGP. There is no setting for the AGP slot speed because it's 8x by default. I'm running a burnin test now with PassMark but it only tests 2d graphics. Can anyone provide insight into the problem before I start RMAing. I'm figuring that it is the video card but want to have less uncertainty before it goes back. Unfortunately, I have no other hardware to test back and forth to figure it out for sure. What are your suggestions and is there any other benchmark or utility software that I could run to try to narrow in on the problem?