It comes down to how badly you want to run new games.....thats the question of truth really.
If your honest with yourself, your current set up will cope with everything bar new games.
Im in the same boat and its tricky because every major component is in a state of evolution at the moment, I dont think theres any one "core" component you could buy today that wont be superceeded in a matter of 3-4 months....
ddr3, new procesors, new graphics and new mobo tech.....
AS some people say on these boards, the only sensible thing to do, is when you current rig becomes to compromised to enjoy, save up a grand and just buy the best that you can at the time, youl just have to live with the fact it will only be state of the art for a few short months.
Im currently building a system(see sig) , of which ive only got the base components which are unlikely to change anytime soon, and in a bizare twist of fate, ive run out of dosh with other commitments, so I wont be able to buy the rest till feb/march next year....
hopefully the market will be a bit more settled by then so it might be a good thing that im being forced to wait, and ill be able to have more confidence in the lifespan of the components......i hope....
Worst case scenario, it will be just as bad as it is now, and ill just have to buy whats hot at the time and make it last as long as poss.
of course theres the other philosophy, of buying midrange components that are significantly cheaper, which makes upgraging/replacing parts much less heart ache, especially when you consider that todays high-end components will be mid range in a very short time.
I dont think there is a best answer, you just hae to buy what you can afford, when you need it, and make it last.