For any mechanical system, you will need to know the mass you want to accelerate, and the force you can impart to that mass. F=ma. For a rack and pinion system, this is not too hard, but it will require that you look at your stepper motor torque curve, and that you know the diameter of your pinion gear.
Let's take an example with our components -- we'll do it in metric so we don't have to deal with slugs as a unit of mass . Our rack and pinion system offers a 25.4mm (1") pitch diameter pinion and a 3:1 reduction. At low rpm's, our stepper motors have around 2 Nm of torque (note, this is important to check on any motors you are considering -- pretty much everyone advertises stall torque, which is not a very useful number). With the 3:1 reduction, this is augmented to 6 Nm. At a radius of 12.7mm on the pinion, this means the system can exert a force of 6000Nmm/12.7mm = 472 N.
So now you just solve F=ma for acceleration. If you have a gantry that weighs 47 kg, you should be able to get 10 m/s^2 of accleration, or about 1g (a N = a kg*m/s^2). If your gantry only weighs 23.5 kg, you should be able to get 2 g's. Of course, this isn't exactly right, as your motor will loose some torque as it accelerates, and there are some small frictional losses in the system, but this is a good approximation for machine design. Also, keep in mind that most people run two R&P systems (one on either side) on the gantry, so they will have double the force to play with.
With a direct drive like you are proposing, your accel will be slower -- you will not have the 3:1 torque advantage in the example above, so your force will also be lower. This still may be workable for your application, but it will not have the optimal balance of speed and cutting force.
Hope that helps!