Shintai,Pinacolada summed it up well here:
So until you really need new computer or you are into multithreaded apps upgrade to quadcore Nehalem does make sense.Otherwise you will pay 100% more for about 10-20% improvement - not worth IMO for typical user it's much wiser to upgrade GPU or wait for cheap SSDs.
Quadcore penryn is good enough to wait till 2H 2009 for 32nm shrink where price/performance ration against Penryn will be much better.Most probably octalcores will be available then.

People are expecting marvels from Nehalem but it will be more evolution than revolution.