You should try to use ( or borrow ) one less powerful GPU ( retail brand) to try, check the linker to see you can boot the PC with retail GPU. Also make sure your MB has newer version BIOS.
If you can boot the PC with other less powerful GPU (retail brand), buy a good brand 550W PSU at least. Otherwise, if you can't boot the PC with retail brand GPU, forget about the new PSU for your gtx 660. May consider to build PC by yourself.