Computer performance is really realitive. The 939 core is far from dead from a performance viewpoint. The AM2's are nothing more than 939's with a better memory controller capable of using DDR2. (As least as far as I have read) That being said, in reviews of the AM2 chips the benefit of DDR2 (now still more expensive than DDR) does not get realized until you get into the higher clock speeds. So the 3800+ 939 will perform much like a AM2 3800+ at least from a noticable/benchmarking standpoint.
In today's PC world, hard drive speed, ram capacity and graphic card capability make more of a speed differance than CPU clock speed. Want to speed up your PC? Drop 2 WD raptors in RAID configuration and watch it fly! Unfortunally, no matter how good the 939 chips were, AMD has dumped them in favor of the AM2's. How long this socket lasts remains to be seen. AMD has some real work ahead of them to catch up with conroe, but I'm sure AMD will take the lead again and then back and forth. Next generation AMD's may or may not use AM2 sockets.
I doubt you would see a "real world" differance in anything you buy that is currently available. (Even more so if it is custom built for you, not a HP or Dell) They are all going to be speedy PC's. (well with the exception of Celeron, yuck, gag) As for me? I upgraded to a 939 even though I knew that AM2 was comming out. I am running a 3500+ venice core with 1 gig DDR. I am really pleased with its performance, it gives my old 3200+ barton a run for its money.
I think that with what you are running a PC upgrade may not help as much as you think it would. I think the graphic cards are where you are going to see the bulk of "playability" of the new games. Nvidia has a new GPU comming out and so does ATI soon. Save your $$$ and get one of those. Don't know if this helps, just my opinion.