I understand your concern about wanting to build a machine that stands the test of time, but there are always new technologies just next door, so if you wait for the latest then something newer will always be on the horizon and you will never get your new PC built.

My advice to you is to set a budget and stick too it, maybe go a little higher if something is a fraction beyond that budget.

At the moment the latest processor to be released are the Intel core 2 duo E8000 series and for the money they are very fast, but since you specifically asked about quad cores the Intel Q6600 is hard to fault, maybe they will release a 45nm version in the not too distant future.

As for GPU's CBR already pointed out the Nvidia9xxx series.

Still a computer using currently released stuff that has a Quad core and 3 way SLi would look something like this:

Case
Power supply
Evga 780i motherboard
Q6600 processor
Processor Cooler for overclocking
4 gig of ram
8800gts GPU
Sound card
Hard Drive
DVD burner

This build would probably set you back a good $1500, probably more and lacks a monitor and operating system but a lot of it is just for show anyways . It also has great overclocking potential at the end of the day it may be a little overkill but then we are looking to the future.