C2D -> newer quad core
 

does it make a day and light difference in Win7? (both overclocked)

I do 20% gaming, 20% video encoding, 60% excel + internet + HD video

quad cores or even the hex core amd will rape encoding... but for gaming you wouldn't see a great increase in speed... and for excel/browsing probably an SSD would be a better upgrade if you don't have one already

Dont buy until December, new I5/i7 chips coming out and there gonna be cheaper + faster ( sand bridge I believe ) and yes it will make a monumental difference, even in games!

will these new chips fit the same sockets or need a new board

You'll need a new board with Sandy Bridge, either LGA1155 (replaces LGA1156) or LGA2011 (replaces LGA1366).

Unless you have an immediate need to require a better processor I'd wait for Sandy Bridge and then upgrade your entire platform. Otherwise, maybe consider getting a cheap (ie: used) C2Q as a stop gap especially since you OC. Those should just about keep up with the current i7 processors for what you do.

that kind of sux, i thought 1366 had quite a lifespan for upgradability