I think multitasking tests done by Anandtech a while ago put the difference in performance when using 200mhz and 250mhz RAM of between 1-10% depending.
I think the highest difference was when runnign Doom3 while encoding or so...
However, that might not be representative of performance for alot of people eg when dual-core games come out there might be next to no difference as both cores might not be running full tilt or more importantly exchaning much data with the RAM (at the same time).
Of course, if the situation could be where say an app or more importantly for many a game could use both cores at 100% and demanded massive RAM access, then the extra fsb would be beneficial - DDR500 then might present some bottlenecking albeit very minor no doubt.
However, more likely is that a dual-core game will not necessarily benefit from massive fsb given the seeming difficult of programming for dual-core yet alone fully utilising it effectively. So perhaps there isn't much reason to worry.
Nevertheless, DDR2 is looking impressive and will only continue to improve and will no doubt build up momentum in the future - DDR is at the end of it's capacity - heck a while back they were only expecting it to scale to DDR400. But, concerning DDR2 I remember reading a while back reading about some 512MB DIMMs that could hit 675mhz at 3-2-2 which I think is impressive and certainly is performance competitive. If my memory serves me correctly they were a new set of Corsair sticks.
As far as I'm concerned however, my interest is more in 1GB dimm sticks and currently, I think DDR is loosely 'better' but this is purely due to their maturity. DDR2 will no doubt surpass the performance of DDR.
But in the end I don't think there should be such a concern or big debate. No doubt certain apps eg scientific might benefit but in real world stuff and particularly performance in games it'll be a non-issue. Current DDR at 500 speeds will no doubt keep a dual-core very happy. Maybe with DDR2 we might see a small improvement over while it'll also be limited in the aps that it is.
As I see it, outside of the fsb, benefits of M2 over 939 will by and large come from tweaking the memory controller and perhaps by slightly better design between CPU & motherboard eg reducing the latencies yet further. In short nothing major especially if you're already running an X2 with fast RAM.
(edit) From a speculative point of view. AMD have cited M2 as being 10-15% faster than same speed S939. Now, if we account for Anandtech tests, if you're running your RAM at DDR500 you might already be getting 5% extra performance (as a conservative number) meaning the difference is between 5-10% with M2. From that let's say with dual-core it's the 10% figure (let's just say the 5% figure is concerning single core), then at *most* assuming you have a 2.5ghz S939 CPU, it means the M2 at the same speed would be performing about 2.75ghz - again nothing earth shaking.
Even so, in that scenario it's likely to much less though as those are assuming pessimistic S939 figures and the most optimistic M2 figures.
We could argue that those 10-15% figures are overrated/overhyped by AMD in order to provide better news for investors and create hype among consumers, in which case performance difference might be significantly less.
Less than 200mhz is by and large insignificant - to shed some light on it the difference between a good and bad overclocking CPU is much more than that.
The only realy benefit to me as far as I can see is that M2 will be 65nm and scale further. BUT given S754 Semprons have been produced on 90nm it's likely 65nm will come to S939 in due time.
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