If we're talking "older than Windows XP" old, then you may want to consider Windows 7 Professional, which has Windows XP mode. But if your games are 2001 or newer, they should be able to work on Windows 7 Home Premium.
Regardless, go with an 64-bit OEM version of Windows 7.
just called up microsoft, said i was a student building a pc, he asked if it was built yet and i said no, he said what is your question i said, i heard you have a student discount for windows 7 and he said that is correct, full version of windows 7 professional or home premium and i said id like professional and my total came to 31.70