Budget On Building Gaming PC Help.

[Takasniper]

Hi everyone I'm kind of stuck on what to buy to build this comp. It'll be my first computer ever that I want to put together myself. What I'm mainly looking for it to do is play Final Fantasy XI at its highest setting and Starcraft 2 when it comes out. Also be able to run 2 FFXI accounts at the same time on dual screens. Don't need to add in keyboard, monitors, XP/Vista, or mouse because I have those. But what would you recommend on operating system XP or Vista for gaming? My budget I'm trying to shoot for is around 450-700. Please let me know what your thoughts are.

PS Looking to keep this pc for along time so being upgradable is a plus.