|When the socket AM2 platform hit the market, the socket 939 CPUs skyrocketed & never came down.|
If you were to compare an AM2 system vs a S939 system where both use boards based on the same chipset (GF6150/n430 for example) & the same CPU (X2 3800+ for example), the performance would be virtually identical. The only major difference would be DDR400 vs DDR2-800 & that difference isn't as much as you'd think.
The cost of upgrading a S939 CPU isn't worth what you'd have to pay. Assuming you have a 3500+ (or 3700+) single core now, you'd need to at least get an X2 4200+ to equal the single core performance.
Here's an X2 4200+ for $130:
For about the same price, you could get an AM3 board & Athlon II X2.