Upgrading this cpu and RAM would serve to highlight other deficits with older technology. Upgrade this unit.
The i5-750 is an amazing quad core that is very (almost strangely) low priced for what it does. It may also be upgraded to a i7-8xx as they share a similar socket, making an i5 a bit more "future proof" than a phenom 2. Also, as the i5 is a more advanced architecture, it may only be matched by a phenom 2 of a much higher clockspeed. Also with your new ddr3 RAM that you'll get with this new unit (that I've already decided you'll get ), look for 1333mhz, as its sometimes a hidden upgrade of the stock 1066mhz at retails, so be sure to ask questions.