machine design help

i have built a couple of machines using screws to move the axis and
what i have found when the table gets long the screws whip and limit
my speed do to that and now i am wanting to build a 48" x 48" ( x
and y ) and playing around with some ideas and thought i would throw
them around here and see if i can learn from you guys experience
befor i spend a bunch of money hehehehhe -- ok here they are ---
using a belt drive system i have found some 1/4 wide timing belts
that have a .08 pitch and claim it will hold to 288 inch pounds the
pulleys for that size belt is .5 with 20 teeth and 1.5 with 60
teeth if i put the .5 on the shaft of motor and the 1.5 and
anothre .5 on the same shaft with the 1.5 going to motor and the .5
looping to the end of the axis travel and both ends mounding to the
truck that rides the rail what kind of outcome with that produce
me ? i am worried that i will be sacrificing torqu with little or
no gain were with a screw of 10 threads per inch i cant get max
speed because the screw whips i acheive 90 inch per min travel
or if i go with the belt system will i still only get 90 inches a
min or less because the motors dont have enough torque now because
the ratio changed so much ? i have also thought that maybe i mound
the motors right on the truck that rides the rails but with tha
added weight to the gantry would that hurt me ? i thank any
engineers out there or just peaple that have allready tried what i
have been talking for sharing there insight in advance