Im getting ready to upgrade my system

It's time to get a new PC!

AMD system: PII-955

OR

Intel system: i5-750

Get whichever has better cost-performance ratio. Generally speaking, i5 is much better in the U.S, whereas, PII-955 is better in most countries.

I would go for i5-750 if the difference of total system cost is within $50 USD.

FYI, i5-750@2.66GHz is better than PII-965@3.4GHz 90%+ of the time. Not mentioned to that 750 can be easily OCed to 3.60GHz and will reach 4.2GHz if you do some tweaking.

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.