Sounds just like me when I put in my 2nd 560GTX. Did the exact same thing.

I was running a 650W PS, then upgraded to a 880W Single-rail XFX PS, and never had another issue.

You have probably hit the PS choke point of deliverable power, and need to upgrade to something bigger. As said before, DONT skimp on a PS...had a very good friend get some off-the-wall brand, and it literally caught on fire, destroying his $1500 gaming computer.