SLI is not supported. A PCIE slot supports a single card, ati or nvidia. P45 supports Crossfire with ATI cards. Ther are different versions of ASUS 'maximus' MBs. This is the P45 version.
Thanks, I already have this motherboard in my computer though, I was just wondering if I could use NVIDIA, as my ATI card (4870 1GB) is probably gonna get an RMA unless I can find out what's wrong.