Your prices are outrageous, you charge more for 1 month rental of a 7770 than you can buy one for ($92 on newegg). The 7850 can be purchased for less on ebay than your one month rental... When people buy video cards they have the ability to resell them in case the coin crashes, if you want this to be anywhere near successful, you're going to have to charge 10-25%/month of what the video card costs, not 125%.
I've decided to offer the 7950 for now. The problem with charging 10-25% per month is that because this a start up I would have to charge users for at least 4 months up front.