besides what (e_dogg) and (xdanisx) have said... I would like to tell you some thing:
keep in mind that the future is 64-bit ( and later for sure ).
so forget the 32-bit OSs, and go ahead with the 64-bit ones.
and your GF's CPU is capable to run x64 platforms, so why not ???
there is some thing I would like to tell you about:
my PC at the office is only Pentium 4 @ 3.4 GHz, and 2 GB of RAM, and they installed Windows 7 64-bit for me on that PC, and I can't describe how improved it was over vista and XP ( on the same PC ).
though I use that PC at work for photoshop, and InDesign, and Win7 x64 is really beneficial with those 2 apps, even with internet browsing and M.S.Office apps, everything changed for me.
my whole experience with Windows 7 64-bit is really amusing over all x64 or x86 platforms I have ever used.