.....I sold off the Bitcoins I bought when the prices were low, and have just over $425 to put towards a 3rd card for my rig.
I've got an extra 550 watts on my PSU, so I don't think I have any real limitations other than temperature or money.
At the current prices, something like 0.55btc each, you could buy 7+ USB erupter miners and a 3-4A d07 usb expander. See current sale under "mining, group buys, "Canary in the Mine"
Then run btgminer under w7 or linux.
Your USB would contribute to hash rate 7*333 >= 2200 mH.
That is four times the hash rate for the price of one card. And yes, you can run the GPU and the USB miners at the same time.
Your two GPU would add 1100 giving you 3300 mH.