A couple of considerations for you, fine sir.

If you're going to build your own miner, you'll need a solid power supply unit. Don't skimp out for a cheapy; you need a good, sturdy PSU. If you're going to go down the path of 2x 7950s, I recommend a bare minimum of 750Watts. I use an Earthwatts Antec 750W PSU; others might recommend more Watts. Secondly, know that crossfire doesn't help your mining - it actually lowers your hash rate.

So you need to tell us your budget. 2x 7950s will set you back ~$450 US I'm guessing and then you'll need to pay for the computer parts, PSU, assemble it yourself etc. Fine-tuned, you'll get ~1200kH/s speeds

You can mine any coin on the market with this; some coins like Primecoin use your CPU to mine instead of the graphics cards but most of the coins you'll want to mine are scrypt GPU based.

Any other questions?