There's more science than you might think to designing a CO2 car. Aerodynamics, thrust-to-weight ratio, surface drag, rolling resistance and friction--all play a role in what makes a CO2 car fast or slow. From aesthetics to engineering, the only limitations to CO2 car design seem to be those artificially imposed by the car's racing class.
The Flying Toothpick
These single rail designs are kind of like old Ferraris or Alfa Romeos, designed to perform brilliantly for short periods of time before disintegrating into their component parts. "Toothpick cars" are simply a pod-shaped housing for the cartridge, and a set of axles connected by the thinnest possible single rail. Like the Top Fuel cars they resemble, toothpick cars sacrifice all structural integrity for the sake of light weight and speed.
If you're going to do a single rail car, consider making the rail a little wider and taller, and carve a channel out of the bottom to save weight and increase surface area for greater rigidity.
Platforms are almost identical to toothpicks in design, except they utilize a single rail that is almost the entire width of the body. The good news is that structural integrity is much improved over single-rails, so your car is more likely to survive the race. The bad news is that that integrity comes at the cost of weight, so it won't be as fast.
Shell cars are bullet-shaped, hollowed-out bodies with the wheels enclosed inside. These are definitely the most aerodynamic of any design, and will usually come to dominate any class into which they are allowed.
This is the design preferred for the top echelon of CO2 racers, and is based on jet-powered Land Speed Racing cars. The philosophy behind shell cars is that CO2 drags aren't really "drag" races at all, as they much more closely resemble jet car land speed races in both principle and vehicle type than any sort of drag race.
In terms of speed, the shell car's only real rival is its pod car cousin. Like shell cars, pod cars have enclosed wheels and axles, but have a body shape optimized for decreased frontal area. Although not as aerodynamically slick, the lower frontal area of pod cars makes for smaller pockets of drag behind the much larger shell car. It's really a matter of personal preference; any aerodynamic difference between the two designs is fairly academic.
If you're going for style points, then a pod car is hard to beat. They can be sculpted into any number of beautiful and flowing shapes, and allow for a degree of personalization that shell cars don't. Although the more complicated shape of pod cars makes them a bit heavier than shell cars, that advantage will be negated if you're racing in a class where minimum weight is regulated.