That. Is a terrible, terrible idea. You've swapped an excellent, high-quality PSU, for a fairly crap, low-mid-quality one. Your PSU is potentially the most important piece of kit in your system, the one thing you don't want to cheap out on.except the 750w corsair psu for a rosewell 750w psu.
There will be at least a small performance boost on the 6-core over the 4-core - worth the $20 extra, I'd say.