I built a rig with a 5870 and a 5850 6 weeks ago, cost me 850€ (including RAID 1 HD config and good quality power supply and motherboard). I have sold BTC for 650€ up to now which is in my bank account. Expect to have my investment back in about 2 weeks (regarding the current price and difficulty).
I am actually also working on the same rig, so it has been a good investment.