I currently Have a Core 2 duo E6550 and it has been fine for the past year, serving me well.
However recently I have gotten into 3d Modelling / Animation and have been thinking about maybe getting a Quad core to cut my render times down.
I have looked around and found the Core 2 Quad 6600 Which almost halves render times in the test programs on the charts on here. However in some games (which I also play) It hasn't had much effect (again going by the charts on here). I am after an answer whether it really would be a worthy upgrade for me for the price also, think its around £150-170, if it is only going to be an upgrade in this one niche area for me, or overall will i see dramatic improvments.
Minor in current games, will show more as games are released that utilise quads. (Supreme Commander, GTA IV)
It will as Sharken says, be mostly seen in rendering so thats a personal decision really. If you dont do that much / doesnt bother you that much then sit on it for a few months, as there may be some juicy price cuts coming soon.
Ye after thinking about it and looking around a little more, Probably best if i wait as my rendering would be the only thing that would really improve and that isn't essential at the moment. If your right about potential price cuts then I will be first in line