http://www23.tomshardware.com/cpu_20...=880&chart=436

Based on what I see in my labs here with Athlon X2 and Core2duo machines and a general guesstimate based on the charts in the link above, I'd guess you'd see about a 15-20% increase in performance with your setup just from the CPU. Upgrading your memory as you mentioned will also gain you a not insignificant gain. I'd guess with both of those upgrades, you'd see about a 25-30% improvement.

This is ballpark based on my experience in the industry, so don't bank on this.