I've been looking into Bitcoin for a while, and I've made very little in my stay in the community. I was lucky to mine for about .75 BTC last time I was mining back in March, but now my poor 5770 just can't keep up.

I'm thinking about investing in possibly 2 or 3 6990's (My current mobo will take 2 easily) and then OCing them to the highest possible hash rate without putting them in danger. According to the Hardware Comparison Chart, someone with 2X 6990s was able to get 1700 MH/s with what appears to be minor OCing. At the current exchange rate, I would on average make 1.24 BTC in 24 Hrs and 37.69 per Month ($632.41.)

Barring market fluctuation and difficulty spikes, I could make the cost of the 2 cards back ($1540) and more in about 2 and 1/2 months running 24/7 (Excluding the cost of electricity.) Does this appear to be sound, or are there details I'm missing out on?