How long do you plan on using the computer before upgrading again?
I build a dual p3 cpu w/ 2gb of memory about 7 years ago and it is still my everyday computer today. That was considered "insane" back then.
I bought a Sony Vaio 2 years ago and I only use that when I can't use my desktop since it can handle running 1 program at a time without being bogged down. I should also add that my wife is on her 3rd computer since I build mine.
The next computer I plan on building will be a dual quad
and that should last another 10 years.