i5 is your best bang for buck processor out there. Top bench marks and right now i think newegg.com has it with a $15 off promo code, so only $205. Very good cpu. If you had the money, i would look at getting a GTX580. for only another $130, its a huge upgrade. What resolution is your monitor though? that is a big thing in determining upgrading to the GTX580.
Only other thing i might add now that i look at it is see if you can find an MSI Twin Frozer II GPU. They are rated #1 right now for cooling and performance.