Not when too many articles agree that single-threaded apps would still suffer at a lower base GHz speed.
By the time all major apps are properly multi-threaded (Snow Leopard just sets the basis for developers to follow), it'll be 3 or 5 years from now and it'll be upgrade time again. By then affordable Pro systems will have 4GHz CPUs and RAM will be cheaper than the sand it's made from.
I also considered the low-end 8-core costing like $300 to upgrade to 16MB compared to the $700 I'd have to pay for the 4-core system. Given prevailing costs, it's still cheaper to go the $640 route (but not by too much and most of what I do wants more RAM; for big renders I can leave the computer on overnight. Or $440 if I went for 3 RAM modules instead of 4 and have a higher memory bandwidth, which I likely won't see a major performance difference in. Again, overnight stuff...)
Either way, benchmarks show even the single quad cremates a 3.05GHz iMac and mine's 2.93GHz. I'll be happy.
My new Pro:
* Xeon 2.66GHz (I did not spend the extra $450 for the 2.93GHz model; a 270MHz difference is 5% - negligible)
* 8GB RAM ($225 and Apple tested it in-house, it's worth the $50 differential after shipping and that gamble there'd be a problem with the delivered RAM from a third party)
* ATi 4870HD video ($180)
* AppleCare 3yr ($200)
$3081 after tax. (If I went with the 2.93GHz unit, it'd be $3600 after tax, and if I went 8-core the price would have been $3800. I cannot afford either of those and with the 8-core, there's not enough to justify it and by the time it is, newer hardware cremating it will have arrived.)