depends on what your using it for.... for any pc to be usable for gaming for more than 4 years... no matter what upgrades you manage to get, that will be a hard task. amd claim their am2+ boards will be compatible with their next gen 45nm phenoms, only thing youll miss out on with am2+ board vers am3 is ddr3 memory, how ever, the 45nm phenoms arent guaruneed to be much faster (if they even get their lol) than the current intel 45nm quads. also its not upto amd wether your mobo manufacturer makes a new bios to support 45nm models.
if your gaming defo go with a faster gfx card, the hd3850 will run all new games, not neceserally the fastest card in the world but it does have low power consumption at idle for a card that powerfull.
if your wanting low power consummption go for a psu which is matched to what your system will consume and look for the 80+ mark. psus are allways most eficcient in the middle of their range so your pc at idle (where it will be most of the time) will not be in a 500w psu's high efficiency band. go for something ~350w if you dont intend to add gfx card and proc with a high tdp.
if your gaming then go for a better gfx card, it will save you an almost instant upgrade, the same goes for the cpu E8400 would be a good choice, if ur not gaming then that set up will be good for atleast 3 years
Blacksi, Shadow -- I'm not so naive as to think that you can truly future proof a PC; I guess what I really mean is
1) Are these likely to be considered decent quality mainstream components for a while?
2) Since I'm concerned w/the energy efficiency, will I be annoyed by these components soon (ie are components with better power/performance ratios right around the corner or a year away?)
3) Does it seem like an easily upgradeable system?
I agree, i would go for e8400 and/or 8800 GTS. If you can't afford a GTS then get an 8800 GT. e8400 is somewhat "future proof" considering its overclockability. You can get 4 GHZ+ on it if you have a good cooler. The rest is fine. If you make these upgrades and don't mind medium graphics in future games, it can probably last 2 years or so.
If you want some futurre proveing than your best choice is AMD since the socket AM2+ is avaliable while Intel is making a change as we speek. So if you want to stick with Intel wait, or just be aware that you buying in something that will be old news soon. But your build looks solid.