Optimal OS for Inspiron 530?

I have a Dell Inspiron 530 with a quad core and 4 gigs of ram @ 667 mhz. Recently purchased a Seagate 7200.11 32 mb buffer, 500 gb sata drive that I would like to replace as the c drive. Currently the C drive is the stock unit that came with the Dell. I plan on installing an OS on the Seagate, transferring data over from the original c and then wiping that clean and using it solely for storage. I currently use my computer mainly for music production (apps include sound forge, reason 3, recycle, cubase, etc) and consuming media (movies, shows, music, etc). Would it be worth my while to upgrade to xp 64 in order for all 4 gigs of ram to be recognized? Would this show a significant improvement in overall productivity and multi tasking? I feel that with the new hard drive installed, my performance will boost significantly. Is this true or will my bus limitation hinder this boost? Bonus points to whomever can find me xp 64 drivers for my m-audio axiom 61 usb/midi keyboard controller.