That PSU will not be able to run 270xs, not sure where you read it could. A new PSU would help you immensely, surely you can afford a new one if you can afford two new GPUs. I suggest http://www.newegg.com/Product/Product.aspx?Item=N82E16817182072
this, it will be able to handle it. Your current is a fairly low quality off-brand PSU, while the other one is a good brand and is 80+ bronze.
Thanks for the advice, but I'm only buying a new card because my old one failed. I would rather just stick with one 270x than buy a new PSU. Most of the wattage calculators suggested my system+two 270x's use an even lower wattage PSU than 550. After a bit of research, I see that the problem is the amperage on the 12v rail.
Do you think even a single 270x would work with my PSU?
My CPU is a phenom II 975 in a AM3 socket, a regular mainboard, I have 1 HDD 1 optical drive and 2 DDR3 RAM sticks, if that information helps.