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 3d acceleration...
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
enjoy!
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..