Personally, I would say look for a used EV PL-20 or EV RE-20.... of course the price tag keeps a lot of people away from them, but they are worth the money.
Also
the Shure Beta-91 is an awesome Kick mic - and I generally don't like Sure mic's, but these are great.
I've got D112's mounted inside my Kicks, and for metal they aren't bad, but once you beat on them awhile (i.e., use them for kicks) they aren't really useful for anything else because the diaphragm gets overworked from the kicks. Half of the time I'll wind up triggering. Just put a small room sound on the Kick sample, and it has bonham all over it. i've got a tape around here where my brother and I were playing with kick drum sounds, and we had one that sounded like the best doubleheaded kick drum boom you could imagine - and the boom was all done with a small Room Reverb on a Behringer multi-effect unit.
The 421 is a killer mic for kick.
Another is the Beyer TGX50 if you can find one.
http://www.nyteowl.com/boutique/TGX-50.htm
I don't know if they still make them - I don't see Beyer Mic's too much anymore. (and for the record the Beyer M422 is one of the best snare mic's ever! It cost me $150 in 1998, but it was WELL worth it.
http://www.cannonsound.com.au/beym422tg.html)
For more click sounds, go with a D112 or an Audix.
The 308 isn't bad, but it would never be my first choice for a Kick mic.
