[Dean]

What do you want to spend might be a better way to start.

Yes, most Dual core speeds are slower, but they are faster due to higher bus speeds (800 vs 1333+) to RAM and Peripherals as well as the ability to do 2 things at once.

Your old system may be Hyperthreading capable, but that just makes it Pseudo dual 1.7Ghz box at best.

Newer games are designed to take advantage of the multiple cores.

A mid range Dual Core with a pair of SLI video cards might be 2-4 times faster running modern multi-threaded games than your current system, though I am far from an expert.