A lot of people around here will recommend the DFI Lanparty series. From what I have read, it seems like a really good motherboard. It looks like it is very reliable, and easy to set up. It also is the best overclocking motherboard available at this time as far as AMD systems go.
As for video cards, eVGA seems to be a good one. I've read good reviews on it, and its inexpensive.
As for the actual card, 6600GTs are a good bang for your buck. 6800GTs are a step up from 6600GTs, and will perform really well.
To find out exactly how much performace difference there is between the different SLI cards, go to www.tomshardware.com
and look for the PCI-E VGA Charts.