No, I wouldnt recommend that. If you had to buy an OS, I'd recommend Home Premium over Ultimate just because it has a "lighter footprint" for a gaming system.
I know lots of people using Ultimate while gaming and not having any problems.
That AlacrityPC utility I linked above can help you shut down processes that arent needed while gaming, and then restart them later. Not required, but handy. You can have custom game profiles that shut down unneeded functions and startup your gaming utilities (like TeamSpeak), gaming device profiles (mouse or gamepad macro files) and start the game in one batch file.