In both cases, define "best" :)
If you mean "fastest", "most powerful", it's easy: a Core 2 Quad QX6850. Will cost you over a thousand dollars though.
For just over a quarter of that, you have a Core 2 Quad Q6600.
Bit slower, but for the money you can replace it with a faster one twice when higher spec CPUs drop in price and still spend less money in total.
It is however less overclockable. But then Intel CPUs are sold at close to their optimum clockspeed, so there's less overclocking them anyway than with AMDs.
Intel does factory tests of each manufactured CPU, and sells it at its peak performance, so a batch of (in theory) 2.4GHz CPUs can contain both 2.2, 2.4, and 2.6 GHz CPUs with the majority being 2.4.
The ultimate motherboard for Socket 775 CPUs is likely the Asus Striker Extreme.
There are of course others, like the P5N32-E SLI Plus.