I agree with Techno, I would get the Asus A8N Premium or Deluxe

. They have the nVidia nForce 4 SLI chipset. The Premium version can switch from SLI to single from software (two reboots needed), the Deluxe version needs to chance a card placed between the PCIe slots (anyway you change this only once, so it shouldn't be a problem).
Even if you're not goint to use two expensive video cards now, in the future you might want to add more performance this way. I prefer only one good video card, but if I add now a GeForce 6600 GT (for instance) which isn't that cheap, in the future I could save and get another one instead of replacing it for a 6800 GT.