Best operating system for a gaming PC?

[shadowtech64]

Getting back to the OP's question...

Between the three operating systems you've mentioned, I'd recommend Windows 7 Home Premium 64-bit, but that's more out of personal preference. The UI in Windows 8 really doesn't work for me. If that's not an issue for you, then you may as well go for Windows 8 now, as it'll have greater longevity. There's absolutely no need to go for the Professional or Ultimate editions of Windows 7, unless you want file encryption or to join a domain.

Windows XP is a great OS as it's lightweight on modern hardware, but as this version is 32-bit and can only utilise 4GB RAM, it's not suitable for gaming. Once Microsoft withdraw support, you won't receive any patches either. Windows XP is great, but I won't recommend it.

To put the cat amongst the pigeons, have you considered a Linux distro, such as Ubuntu? If you're using Steam as a gaming platform, Valve are making serious strides towards Linux with their Steam Machines. Granted, not every Steam game is, or will be, compatible due to third-party DRM, but this may change in the coming months and years.

Anyway, if your budget allows an OS, go for Windows 8 if you can live with the UI.

windows ultimate and pro give you more than 16 GB of memory