By the time Nehalem is released there will be another processor on the horizon, and another, and another. If you want a new computer, look in the now, or maybe 1 month from now, but no further than that. If you keep waiting for something better, you're never actually gonna buy a computer.
I know there will always be something better on the horizon. My current pc is only 2.5 years old but it's going to be a hand-me-son to my son since he spends more time on it then I do. So I'm not in a position when I need one right away, I can wait the 6 months or so if it's going to be much better than today's quads.
I'd say Stickciv is right. The next gen processor schtick almost seems like a pricing racket.
I'd probably wait until both come out on the market a few months each, since it seems they're both quadcore 45nm cpus. I think it will be an issue of features for price, since the Nehalem will bring back hyperthreading and they're similar.
If you check the quad vs. dual thread here, it seems to be a matter of what you use your desktop for. Do you video edit or leave two dozen windows open at once? Do you just do a little bit of everything?
If you're a yes to the second answer, I'd go dual-core and possibly Wolfdale (3ghz typically overclockable to 4ghz), though who knows what will be on the horizon in six months. .