this is what i will tell you about nvidia drivers...
when you install the drivers, and after you install them the machine boots up without a problem, then you do HAVE
if you read the documentation about the nvidia drivers you'll see that this happens on some distros, and its irrelevant, cause like i said if the drivers install, then you have it, plain and simple, no arguments
so in answer to the question, you don't enable it, cause it already is
as of ut2k3 i have no idea about the game as i don't play it, but i assume it is just like any other game, and it has an options menu, and a video submenu to change different settings, which you could try if you already didn't...and i don't know what desktop environment your using, but as of the refresh rate, i guess you could change it, but that will depend on what your monitor can handle, and you'll have to go into your machines settings, like control center or something...again i don't know what enviroment you use, so i can't tell you how to get to it...but just use my keywords and i'm sure you'll stumble across it..