GA-P55A-UD4P DDR3 RAM Requirments

I just purchased a GIGABYTE GA-P55A-UD4P with an INTEL i5-760.

I plan to use XP x32, XP x64 and Windows7. All on different HDDs of course.

I am going crazy trying to figure out how much RAM to install, what speed, what voltage, how many modules.

I am not worried about overclocking at this time but want to keep the door open if I choose to do so.

Things have changed a lot since I last built a system.....