Do a lot of reading in this forum, there are a lot of threads that go into details on rigs and how to set them up. At the current difficulty (I'm assuming you're wanting to mine bitcoins), realize that you will more than likely not get your money back.
Couple things I would suggest - Start with building a single rig, and plan it out before purchasing. 2 cards will work easily on any newer motherboard/processor and in a regular PC case, but anything over that is going to require more work, like a crate rig, multiple PSU's, risers, etc. Make sure you check the Mining Hardware charts at https://en.bitcoin.it/wiki/Mining_hardware_comparison to see which cards offer the best performance for the money (7790's, 7950's are good value right now). Occasionally good deals pop up here on the forums (the recent rebranded 5970's for $129 was one such deal, I only managed to get 3 of them).
Good luck.
Personally, I'd recommend getting an ASIC when they come out, as the difficulty is getting ridiculous. Couple things I would suggest - Start with building a single rig, and plan it out before purchasing. 2 cards will work easily on any newer motherboard/processor and in a regular PC case, but anything over that is going to require more work, like a crate rig, multiple PSU's, risers, etc. Make sure you check the Mining Hardware charts at https://en.bitcoin.it/wiki/Mining_hardware_comparison to see which cards offer the best performance for the money (7790's, 7950's are good value right now). Occasionally good deals pop up here on the forums (the recent rebranded 5970's for $129 was one such deal, I only managed to get 3 of them).
Good luck.
