Home Premium's got a 16GB cap on how much RAM you can use, and Windows XP mode is only available on Pro or higher. Networking features also differ, etc... but only you can tell what you'll need. The Wikipedia link below goes into much more detail than the Microsoft one. The differences really don't matter much to gaming alone.
http://windows.microsoft.com/en-us/w...oducts/compare
http://en.wikipedia.org/wiki/Windows...mparison_chart