building from scratch.
Any idea how much you're looking to spend?
2 x 5870's put up better numbers than than 1 x 5970 but use more power since there's 2 cards. 5970's are more than twice as expensive as 5970's, even on ebay. So for those you are looking at a power vs price trade off.
6990's are newer and roughly the same price range as 5970's. Drivers work on Windows and there's mixed success on Linux. There's probably room for performance gains in future drivers.
You have to decide how much you're looking to spend, which cards and how many, find out what power supply will support those, a motherboard with enough slots to support the cards (up to 4 cards, but it's not cheap), etc.
If the box isn't going to do anything other than mining, you can get away with the lowest power and speed CPU for the board you pick, and 1-2G of memory since the CPU and memory aren't used in the mining process, only running the system hosting the cards.
My Ubuntu installation is less than 4G, so you can use a decent size/speed hard drive or small SSD.
