If you want to do the Army and don't want to do JAG, do it first.
I would seriously suggest checking out the ROTC program at your school. A scholarship person there will be able to answer any of your specific questions in detail. ROTC will pay for under grad and then when you deploy after graduation you will be available for the post 9/11 G.I. bill which would pay for the majority of your graduate school. I am currently an ROTC cadet and will commission in May. I plan on serving in the Army reserves while attending law school the transferring over to an active duty JAG after law school graduation.
If money isn't an issue right now, honestly I wouldn't bother with ROTC. You can go through OCS after graduation and you will not have to sacrifice your summers now as well as missing out on the military science classes and the FTXs that always seem to occur when you have something better to do.