Well, first let me address my previous LGA775 comment.
I realize that the prices for P4 chips are slightly more than 754 (about $25 or so for equivalent speed/rating). I also realize that A64's are marginally faster in some benchmarks than their equivalent P4. However, the price difference and the performance difference is relatively small overall (IMO), and you get the opportunity to use the new tech as it arrives, which means you should be good to go with the same MoBo for a few years. Now, I haven't heard anything about the LGA boards/chips being unstable, but if that is indeed the case, then you'd certainly have to consider that aspect when weighing your options.
When you start comparing the LGA775 to the 3500+ (@ the current $450 price for the 3500), then the LGA doesn't look expensive @ all. This is especially true when you realize you can get something like a 3.0 GHz LGA chip for $205. This makes it very easy to enter into the 775 socket without spending much money, and gives you very good performance and a very good upgrade path (aside from the 1066 FSB issue, which I think is the chipset I would wait for....maybe). Anyhow, my overall point is that Intel has given the consumer the ability to jump to their latst MoBo and associated technology, at approximately the same price/performance ratio as their previous generation. AMD has not accomplished this, and their newest socket is far more expensive than a similar performing example of their older tech. I feel that this is something AMD needs to address soon. First, by lowering the price of the 3500+ to $300-320, and second by adding some lower end 939 chip(s) soon; perhaps something in the $200 range (maybe a 3100+) that would make the entry into 939 very easy.
Now, as far as upgrading goes, I agree that it is difficult since things seem to change so quickly. When I bought my AXP 1800+, I had hoped to be able to upgrade the CPU only after a couple years, but due to numerous changes that is not possible (at least with my Mobo). So, what I would do now is buy a chip @ the low end of a platform, with the hopes of upgrading to a higher end chip sometime in the future. On 754 that would be maybe 3000+ now, with a 3700+ in the future (not much of a change IMO). On 939 that would be 3500+ now, with maybe an FX 57 in the future (I think this would be a bigger change).
Anyway, I'm still holding out till december (when my current computer hits 3 years old) and we'll see how things look then.
-D'oh!