|500 series board? Obviously it will run a Phenom II but since it's not an AM3, it doesn't fully support the technology.|
nVidia is currently pushing out 900 series socket AM3 boards with triple SLi & DDR3 support. There are very few boards to choose from though because nVidia has virtually dropped out of the motherboard picture. For example, if you check Newegg, there are only two AMD boards based on the 900 series, but there are over 150 boards based on AMD chipsets. Ditto on the Intel side of things.
IMO, SLI & Crossfire configs are a huge waste of money, but "investing" $800 +/- for two video cards & a PSU for an aging system is totally ridiculous. I highly doubt you're going to want to hang on to that outdated board "for the next few years", at least not for your primary machine.