are you asking why a 2.8ghz would be more expensive than a 3.0ghz? If so it has to do with how many cores it has, the more cores the more the computer can handle.
Explained:
http://www.techterms.com/definition/clockspeed
DDR3 ram will not work with socket 775 motherboard. Do you already have 775 motherboard or are you planning on buying one ? CPU can only downclock itself if it doesn't need all its potential so you save energy but if you want to get more of it then you will have to manually overclock it.
Oh ye LOL im dumb i was thinking of triple channel memory :X
ah get it now.... thanks =D
another question....
If a game optimal spec requires like at least 3Ghz C2D CPU , can a 2.6/2.8 C2Q / i7 runs the game smoothly??or shud we overclock the CPU to the 3Ghz requirement???