Personally, I would wait about a year after the launch of the new AM2 socket by AMD. You may wonder...why? Well, experience tells me that when a new socket comes out, a first generation of motherborads comes out, being these mobos not good for enthusiasts since they have bugs and are not good for overclocking if we compare them to the 2nd and 3rd generation/wave of motherboards that will come out a year or so after AM2 hits the streets.
Think that DDR2 modules are still rather new, tight latencies like 2-2-2-5 1T will not be seen for a while, maybe we´ll have to wait for 6 months at least to see a DDR2 4200 or 4300 modules running at those tight timings with a normal DDR2 voltage.
Right now, the tightest DDR2 Ram modules are the OCZ DDR2 PC2-4200EB Platinum Ltd. Rev.2 with 3-2-2-8 1T timings at 533Mhz (266x2) and they cost 375$ for the 1Gb (2x512Mb) set, that's rather expensive if you compare its price with current top notch DDR modules prices. You may think you could run them at 2-2-2-5 1T, huh? No, although you could get them on your system, you wouldn't be able to run them with CAS 2 at a lower frequency because mobos don't support it for now, we´ll have to wait.
Also consider that the current AMD Cpu line-up does not like the loose Ram timings DDR2 provide right now, time will solve these issues I hope, IMO, you should wait at least for the 2nd wave of AM2 motherboards from a vendor after socket AM2 appears.
I will wait about a year with my 939 setup, upgrading the cpu to Dual core, for example, and the video card to the new g71 gpu from nvidia...or the DFI SLI-DR to the SLI-DR Expert...that kind of upgrades, but maintaining the socket 939 platform and DDR ram.