FSMO roles, moveing them

Hello! I have a question related to Active Directory and their FSMO roles.

I have two physical sites (about 2 miles apart) and have the PDC (2008 R2 VM) ay my location and a BDC (2008 R2 Physical server) here as wall. At my 2nd site, I have a BDC (2003 VM) there as well. I have just started here at this company and noticed that one DC is hosting all the FSMO roles.

I have quite a large network and per best practices, I am looking at moving the roles between two DC's.

My question is :

What roles need to remain together?

What DC's should I uses? (Should the roles remain at one site?)

Any info would be helpful

Thanks!