Just my pint into the fire: 
X2 4200+EE & GF6150 board (MSI K9NGM2-FID) 
$240 (170+75) 
E6300 $ G965 board (ASUS P5B-VM) 
$285 (185+100) 
Conclusion: 
Anything cheaper is K8 vs. Netburst so Intel is no contender. 
Anything more expensive is K8 vs. C2D clock/clock so AMD is no contender (4800+65nm is more expensive than E6400 it matches by perf.) 
For decent IGP-free boards the difference is comparable. 
So, going for 
stock performance the choice is simple: 
<$300 for CPU+MB combo go AMD X2 
>$300 for CPU+MB combo go Intel C2D 
for overclocking: 
<$240 for CPU+MB combo go AMD X2 
$240-$285 -> wait and then Intel C2D 
>$285 for CPU+MB combo go Intel C2D 
for power consumption (i.e. PC's for office use): 
AMD X2 3800+EE to 5000+EE (anything above or down is a waste of money in this case) 
for single core: 
<$190 for CPU + MB combo -> go AMD A64 
$190-$230 for CPU + MB combo -> go AMD X2 at $240 
>$240 see dual-core recommendations 
That IMO sums up the whole Desktop PC market as of now.