Most definitely buy a beefier power supply...your machine may not even boot otherwise. I'd say go above what you need, just because you get a PSU rated at 300W, it doesn't mean you'll get 300W out of it (unless it's a really good brand).
I had to upgrade mine when I got a new mobo and 9800 pro, and went with this Aspire:
http://www.newegg.com/app/ViewProduc...148-005&depa=1
It's not a brand name everybody instantly recognizes as one of the biggest & best, but the reviews were great, the price is cheap, and it looks & runs fantastic.