I hate that everyon is asking this question now, but no-one is asking one of the most important questions to give a proper answer: What is you upgrade schedule?
By this, I mean if you get a new system now, how long will it before before you buy another system afterwards / give the system you get a completely new overhaul.
Also, there's no direct evidence for what the previous two guys have said, it's all based on rumour and speculation. Firstly, there's no reason that AMD can't put 4 cores into their current socket 939 package, and sample have bearly been shipped, let alone benchmarked to be able to conclude what kind of performance difference there will be.
The largest difference that'll happen between socket 939 and AM2 is that the AM2 chips will have a memory controller that'll work on DDR2 RAM, not DDR. DDR2 latencies have finally dropped to the point where they now offer a competative advantage over standard DDR, so this is why AMD is now choosing to switch to DDR2, unlike Intel, who jumped on the DDR2 ship right away.
Now, the reason that I ask what your upgrade schedule is like: It'll still be atleast a few months before we see the first AM2 solutions reach the retail market, and I'm betting a couple months after that before we see AM2 motherboards that are as full featured as current 939 offerings.
The performance difference between the initial offerings of AM2 and current 939 will be minimal (Probably CPUs clocked at the same speed, jsut using DDR2 RAM instead). But over the course of a 6 months to a year, we'll probably see faster AM2s get released. However, AMD has stated that they'll stop 939 production by the end of the year. The current FX-60 chip is expected to be the best performing 939 chip that we'll see, with the possibility of a speed boost to the Sempron line, but that's about it.
The point of all this: If you upgrade your machine once a year, you should get a socket 939 solution now. It's a mature platform that offers proven performance. Then, by this time next year, AM2 will have been out there for a good time that issues that will be found with it (You cannot assume that there won't be any issues with the initial offerings), and will have been addressed. I'd probably recommend this same action if you are on a 2-year rotation, as CPUs are not the current bottleneck of computers.
Conversely, if you don't plan on getting a new computer for 3 or 4 years, then it will might be worth considering waiting for the AM2.
The point to remember with all this is when do you expect to want to upgrade your CPU. AM2 will not have any effect on the SATA standard for hard drives, PCIe connections for graphics cards, the ATX standard for your PSU and case. The only components affected by it are the CPU and the RAM (if you consider AM2 as a function on your motherboard choice). And even with the RAM, DDR RAM will still be available for years (Look how long SDRAM has stuck around).
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