Your board does indeed support the Quad Cores. You can find the whole list here:
http://support.asus.com/cpusupport/c...Language=en-us
You cannot use SLI on that board. But we don't recommend SLI anyway. The bridge adapter comes with SLI motherboards. And I believe the newer boards don't require the bridge, it works via software.
If you're looking to overclock and have a new CPU, I'd bump your RAM up a step to DDR2-800.