I dunno. VIA conflicts with GeForce I know but since you have the nForce 4 chipset (good man!!) I'm quite lost as to what might be causing this.
There is a possibility however remote that your hardware is just too damn new. Sometimes they release these things w/o having good stable drivers for them which causes all kinds of weird issues. That would be my guess here. Also you may want to call ASUS. Some of their past boards have had issues with certain cards in certain slots. I think they cleared that up some time ago, however, due to bad press, bad ratings, and bad rep from people. Their boards are the fastest, the most stable, and the best around if you ask me though. I won't touch anything else. ASUS slams hard and you can hit them extremely hard in any game and they just keep on cruising.
Perhaps you have a bad board. I'd contact ASUS for sure on this one and talk to their level 3 support. In other words, get past the minimum wage 'read from the card' morons and try to talk with a technical engineer. They are very helpful.
From the first post it seems that this is more related to drivers than anything. I've had San Andreas do some weird things on my rig as well so I'm not surprised at that. Either update the drivers (if there are any updates), wait for a new one, or search various forums for similar issues. Again your hardware is very new and while that means you are the uber cool gamer dude who gets to host everything, it also might mean a host of new problems.