H4 bulbs work fine for me - I haven't run into any heat issues or wires melting if that's what you are asking

You will need a H4 bulb converter and have to remove some of the tabs on the bulb to get it to fit.

I was also curious about the HID route, and while they may work/fit, getting the bulb to aim properly in the stock headlamp housing could make your night time driving unsafe. Also getting a "good" HID kit is like buying chinese ABS fairings, kind of a crapshoot

More info here:
http://www.st-owners.com/forums/showthread.php?t=17579
http://www.casporttouring.com/cst/motorcycle/17120.html