First, buy the UCL mentioned above, no matter what.
Then go with the 3-to-D adapters and 6 cell bulb if you want to get the brightest "easy" mod. Some would even say to overdrive a 5 cell bulb, just keep extras handy because they won't last long.
But, the route I'd take is to just purchase an EverLED replacement and drop it in. You can keep the 2D batteries or do the PVC mod to make better use of the dead space in the mag's tailcap that is normally used to store an extra bulb. This won't be as bright as the first mod, but it will last a heckuvalot longer on one set of batteries AND you'll never blow the bulb. Should make for an outstanding (and indesctructible) general purpose/utility light. It will be a whiter, better quality beam that should be roughly comparable to the stock bulb.
The next step up would probably involve some basic soldering skills and the purchase of a "hotlips" heatsink, "bad boy 700" or "mad max+" converter and a 1 watt luxeon star (driven by the 2D cells). /ubbthreads/images/graemlins/grin.gif
Jon