Some good advice given here already. As you probably figured out by now there are several ways of doing this - it all depends on your preference.
Another option is to get the proper heatsink, LEDs and reflectors, wire them in parallel and use 3 NiMH C-cells. You will need to modify your tailcap but it's not too complicated (remove the anodizing from the inside, trim the spring so that it fits into the smaller cavity). You will also need to sleeve the battery tube with something to prevent the batteries from rattling around (hose, rolled up paper, old mouse pad, etc). The output isn't regulated but with NiMHs it's flatter than alkalines (it would not be very bright with alkalines anyway) but it's simple and sometimes simple is good.
Having said that, I love my tri-lux with Shark converter and 6NiMH batteries. If I were building one now I would definitely consider the Seoul P4 LEDs or Crees as others here have suggested. If you get urge to go one step further, you can re-wire the LEDs in series and use a converter as others have suggested. Drivers like the Shark or Fatman will also allow you to fit a potentiometer so that you can adjust the brightness which really increases the usefullness of the light.
If you have any other questions, just ask were all here to have fun and share in this hobby.
Oh, and welcome to CPF and the joys of modding - and hold on to your wallet.