Voltage to steppers

I have a home-grown machine that uses small 55mm 12V steppers. I have
driven them with an embedded controler & Allegro driver chips, which
make it fairly straightforward. The setup uses round ground thompson
shafting and linear bearings and moves quite freely, however, the
little motors only have enough torque to drive consistently in either
1/2 or wave drive. I would more importantly like to see some more
speed. during free moves...it doesn't need torque beyond what is
required to move itself around.

From what I have been reading here while lurking (from people like
Dave M, of the Basic Stamp list - Hi Dave!) it seems that it is
common practice to drive steppers above their rated voltage, is that
correct?

If so, what will driving my 12V motors at say 24V do? It seems you
guys were talking about 10X rated V, so I'm wondering what 2X will
do....

Perhaps I should just change motors. Of course they will have to be
inexpensive say $20- and with 1/4 inch shaft. If I need to go over
the V rating of the driver chips, I may have to re-think everything...