If I remember correctly, 6500 used an Octalink-style bottom bracket, so you'll need something like this:
http://www.universalcycles.com/shopp...7&category=347
You'll need to make sure you get the right size. A 90's Trek is probably a standard 68mm English shell. Note that you'll probably also need the appropriate splined tool for isntalling the bottom bracket, something like a Park Tool BBT-32C.
The width of the spindle will depend on whether it's a 2 (109mm) or 3 (118mm) speed crank. You can find most of the necessary sizing information on Sheldon's site:
http://www.sheldonbrown.com/bbsize.html
(scroll down and look for Ultegra 6500)