Well first off the C2D has two cores on one die so there would be CPU's running at 1.86ghz inside of it.
a 2.6ghz Athlon would actually be about as fast as a 2.1ghz C2D but the Core 2 Duo still have two cores running at that speed.
I want you to understand that AMD actually isn't better for the money when you get that high up in the price range. You can also overclock Core 2's to amazing speeds to where AMD cannot even begin to touch. A 4.0ghz E6600...would need about an AMD X2 running somewhere around 5.2ghz to compete which AINT GONNA HAPPEN.
My $180 E6300 is upped from 1.86ghz to 3.22ghz. Also what do you mean??? There are many many good Intel mobos for C2D.
The Biostar P965, MSI P6N, DS3, P5B-E, P5B Deluxe, the P5W just to name a few.