So this summer, I might be recording my band in my basement. We plan to do guitars/drums/bass/vocals into an m-audio interface and then into garageband, each tracked separately. I don't have the cash to upgrade to any nicer programs. Since guitars and bass will be pretty loud in the recording, is it absolutely necessary to get something to bring down the level of the guitars before the signal goes into garageband? Or should I get a preamp involved? (I do have one that I guess could work.)

Also, are there any tips you guys can give me for getting that good stonerish atmosphere down with what the room and equipment I'm working with?

no need to turn things down just put the mic in a good position in the room and record.

Your doing a good thing by recording separately. at that stage, focus on a good sounding signal and capturing some of the room. a loud recording should not do any damage as long as you keep a safe distance from peaking the signal out or clipping.

that's pretty hard to do if your using a good quality mic ( Sm58 etc ) Mic's gets dropped right in front of a speak cab and put thought the PA everyday without clipping the input at the mixer.

BUT if your clipping out the level in Garageband from the M-audio ? maybe use the internal mixer in the M-audio driver ( in its settings ) to turn it down alittle, but again I do not think you will clip out with a good mic ? unless your playing next to a airport and need SUPER volume levels.?
how many quad boxes & total Watts of the amps are you recording ?