Yeah, what he said, except that technically, we're talking about "tread"; q-factor is a bike-fit term that Grant P co-opted to describe tread in the 90s, during his stay at Bridgestone, pre-Rivendell. MTB and BMX cranks tend to be wider between the pedals, to clear the fatter stays associated with >2" tires, and they also oughta help increase the clearance for your chainguard. Keep in mind, however, that mtb cranks will almost certainly give you a higher chainline figure, which will cause other problems if not properly addressed.
I couldn't believe i beat you to this one so i did the best I could..
I just found out about all this a few weeks ago when I installed a set of SRAM Force road cranks on my 2000 Schwinn Panther and ran into the same thing.
Ended up getting this set of Race Face Chester cranks. They have a bit of a dog leg to them.