I have project "Deep Thought" in progress. The building of my first home made machine - dual core chip, 8gB RAM, 2 x 512mB video cards, 2 x 500gB HDDs (RAID). 64 bit Vista Ultimate.
Vista will run fine with 8Gb of RAM.
2x512 Graphics cards will suck juice from the 12 volt rail like it is going out of fashion, so make sure that whatever power supply you buy will be Sli ready. If you want to future proof look fo a 1000W model.
You can pay a hell of a lot for a case as this will help with cooling, some of the more expensive cases are machined from a single bloc of alloy.