I5 750 & Core 2 Quad Q9550

i5-750 by a decent margin. However, if you already have a Q9550 I wouldn't run out and by an i5 just yet. But if you're in the market for a new system, you should NOT be considering that Q9550.
 
If you're building a new PC, then i5-750. Both will do an excellent job though, but the LGA775 is a dead socket for upgrade.

OR if you wanna upgrade your CPU for games, DON'T waste your money and keep the Q9550 for at least a year. Better buy a new GPU.