just a note for anyone else building a SLI rig with the ASUS mobo....to fully enable SLI, you need 2 GPUs (duh), the SLI connector (again, duh). With the ASUS mobo, there is a card (between the 2 PCIe slots) that you have to flip. Then you have to enable SLI in BIOS. Install WinXP and the latest Nvidia drivers and right click desktop and click properties. Click the settings tab on the top. then click advanced. then click the tab labeled GeForce model_name (model name is the name of your card...the GPU, that is) then a little window will appear to the left on your screen...click SLI multi-GPU and check the box that says "enable SLI" this may require a re-boot.
note that all prior info is in the manual...but the next step IS NOT... found that out the hard way 
Click the performance thingy to the left and check the box saying show advanced settings....Theres a scrolable box in the middle of the window...scroll down and select SLI rendering mode. make sure it says multi-GPU rendering...mine said single GPU rendering...if that is the case, click the arrow on the box and select Multi GPU rendering from the drop down menu. click apply and your system is fully in SLI mode..at least to my knowlege (after the last step I got a boost of 3000 in 3DMark 03!! this is a majorly important step)