If u replace mobo and CPU, u need to activate Vista again.
So if u don't have a retail Vista version, the old Vista will not activate, because it is OEM, that means to get a new OS, so it is easy to get the W7 OEM for $99.99
Bottom line is, if u will have to purchase a new OS, just get W7.
I got vista with my computer when I bought it a number of years ago, and got the discs with it.
U have OEM, so u need to get a new Windows.