I went from an Athlon X2 6000+ (3.0ghz dual core) to a phenom 9600 (2.3ghz quad core) and saw a SIGNIFICANT performance stabilization. Everything ran tolerably on the dual core system but I would find myself waiting when the systems processor choked completely. Moving to the 2.3ghz quad core I had slightly lower performance on the main (still quite playable) and that performance was the same in almost any situation.

I can tell you that since I started 5 boxing I've gone from a 3.0ghz dual core with 4gb ram and a 7900gt to a 3.0ghz quad core with 8gb ram and an 8800gt, and by far the best bang for the buck was the 8gb of ram (of course reloading the OS to go 64bit was a PITA but so worth it in the end.) The 3.0ghz quad core chip REALLY made it great as well, and for the price you can't beat it.