If you're running a 64bit processor, then a 64bit O/S is for you. If you have an old 32 bit processor, then obviously go 32bit. However, If you're planning to upgrade from 32bit to 64, then get a retail version of Win7 & not OEM, and again, go 64bit. As 'Hex said, you'll need it to address the ram you'll need. 32bit windows will only ever address 3mb ram, & that's with using the 3gb switch (otherwise only 2gb will be used).