Conroes = More powerful per clock, lots of OC room
AM2 = Cheap as chips after price cuts.
A)If you're overclocking and you're thinking of $400 for the motherboard + CPU then you want to go Conroe.
B)If you're overclocking and you're thinking of less than $400 then AM2 becomes competitive.
C)If you're not overclocking and you're spending less than $400 then AM2 is the best option.
D)If you're not overclocking and you're thinking of spending more than $400 for the motherboard and CPU then Conroe is the only option.
A) = Gigabyte P965 or other board that overclocks well, with a 6300 or better means you're looking at a 2.6Ghz OC or greater (6700 level of performance, ish)
B) = Under $400 you can't really afford a decent core 2 overclocking board and a 6300, as such your options are limited to AM2 with the 3800X2 as a fantastic choice at a very low price. It was a good performer a few months back, now it's just as good but a lot cheaper.
For non overclocking boards you're looking at around $100 for AM2 and $140 for Conroe (roughly). As such if you're going AM2 then a 5000X2 setup costs a stunning $400ish, which is about the same as a 6400. Under that point your options in the intel camp are pretty crap. Over $400 then you're looking at a 6600 which is going to be faster than a 5000X2 by a good margin.
All of that aside all you need is a $100 AM2 motherboard and a $120 3800+ (single core) for anything short of a X1900XTX or 7950GX2 (even for the games that can use dual core, the vast minority) to avoid being CPU bound.
(Anyone who buys a FX-62 needs thier head looked at. A 6800 is crap price/performance but you accept that for the top end performance, but a FX-62 is even worse as it can't even supply that top tier performance at all)
Corrected due to my own stupidity and the poster below's help.