if you already have a mobo that supports the Core2 family, why change the motherboard?
you won't really gain much benefits from changing the motherboard only.
if you want a new motherboard you should go with a newer up to date one and get a core i3 or something.
if it supports ATX mobo's it should fit most of them.
just note that there are some ATX mobo's that is a little wider then native ATX and is still called ATX. (i am not talking about ATX-E)
a good example on one of those is the Rampage 2 Extreme and Rampage 3 extreme. (properly also the R4E)