Show me how you got your 500% and I'll send you some cash and you can keep half of the ROI. Scrap that. I'll just buy more coin.
I bought 1Ghash that pulls 900W for $800.
go to
http://bitcoinx.com/profit/index.php and plug in those numbers for a 12mo period. The calculator counts your hardware as a cost, but it's really more of a capital investment, and should not be counted against ROI.
Now 500% ROI is pretty sweet, but this is a risky investment. My miners would go off tomorrow if my countries government decided that BTC was a criminal enterprise, and I would be left sitting on unproductive capital. I would not add capital to this market with ROI under 300%, and I don't see how any business person could rationalize such an investment. Perhaps I would change my tune in 12-18mo, but not in the short run.
That calculator does not take into account future difficulty. In a month, you could be only getting a third the bitcoins you are now. Make sure you factor in difficulty. As a general rule of thumb, the bitcoins you make in the first 10 days are 1/3 of all the bitcoins you are ever going to make.