Or, what you could do with that $500 is get a
cheap C2Q, pop it in your current board and OC it to 3.2Ghz+, sell the 9800 GTX+ for $100, then spend that together with the rest the money on a
5870. As far as gaming performance is concerned, the rig with this upgrade route will easily destroy the i5 750 + reuse 9800 GTX+ combo that Raserei408 mentioned above.
However if you don't do major gaming (only somewhat), the i5 750 + reuse GPU idea isn't bad.