Buying any Core2 now is just plain stupid, because it's a dead platform costing far too much.
The same is not true for the Phenom II. The 965 might not be as fast as an i7 when released, but the 975 probably will be. We're talking 4 months and I can *still* plug that cpu into a 2 year old mobo.
As for your RAM point, well there isn't really one to be made. The benefits of DDR3 over DDR2 are so minimal that it's pointless right now. However, if I did decide I wanted DDR3 in the future guess what? Yep that's right, I could just go and buy a DDR3 mobo and plug the SAME chip into it.
Finally, if you think you will be able to plug the top end intel into your current x48 mobo in 3 years, I'd really love to see why you came to that conclusion. The i7 is no more future proof than any cpu before it, if you believe otherwise then you are a total fool who has fallen for all the (multi)threading garbage.
Did MMX make pentium 1's last 5 years? No. Did 64 bit make Athlons last 5 years? No. How long have dual core's lasted in reality? 3 years max? Nobody with a brain would buy a dual core now.
Yet you think the i7 platform will last 5 years? Wakey wakey, we're in the real world now. The i7 has 2 years from now at an absolute maximum and that's only if intel don't decide to change their minds with their westmere's and opt for yet another new socket. It's not like they haven't done that before, is it?