You have to take many factors into account if you wanna make your car go really fast. And the motor alone isn't the unique issue.
For the motor itself, you must consider:
1. The Round per minute (or RPM) rating of the motor. This is the RAW speed of a motor alone under its optimal voltage (let's say 3V here)
2. The Torque of the motor: your motor may go fast alone, but maybe it won't have enough torque to move the gears.
3. Power Consumed (how many Amperes it'll need to run)
Then, you'll have to check the gear ratio of the gearbox, in accordance of the torque of your motor...And finally the battery pack (and electronic boosters) you use, to give your motor enough power (voltage AND intensity).
You'll find a very good page with a lot of informations (dedicated to Mini4WD) here:
http://www.geocities.com/alotstore/tips.html
And remember that motor class for Race-Tin is "130" (useful if you wanna buy a motor from an electronic shop).
If you can read french, you may find some info on my site (a compilation of all info I found about Race-Tins):
http://rc132.free.fr