The TNT is always the best solution up to date!
a 3dfx add on is quite senseful if you want to run older glide games, too.
But I wount buy a 3dfx board if you buy the TNT. I f you still have a 3Dfx voodoo or voodoo II, its is not bad for the older games in glide.
buy the way, the TNT is always faster than the voodoo and gives you much more iage quality!