do whatever you want to but at your own risk.... even cm extreme power plus 500w is better than the psu you got....
which gpu are you planning to fit in????
I'm not sure if I'd buy one. At the moment, ATI 5xxx Linux drivers are not mature enough and getting a 4xxx won't be of much use to me. (Someone told me to try DirectX 11)
If I delay getting the 5xxx series by winter or spring (most probable timeline for mature 5xxx support on Linux), I'll get a new PSU along with it.