A fuller explanation of regulators:
The main regulator on any marker setup...is the one that the co2/air bottle screws on to. In the case of air, the regulator uses a series of springs, seals and a piston (generally) with a valve, to reduce the output from bottle from 4500 psi which (unless you have a Mag!) is going top blast the marker into little peices, down to a more useable output preassure.
In the case of a preset Reg, this would be 450 to 850 psi..or anywhere from 200 psi up to 1000 psi for an adjustable Reg.
The better quality (not necessarily more expensive) system will maintain a constant supply of regulated air at a constant preassure (almost)...this is a contributing factor to accuracy of the marker.
The other Reg you may have (many do) is the Secondary reg, often verticly mounted to act as a foregrip...this will reduce the preassure from the main Reg form say 600psi, down to the working preassure of the marker, say 325psi.
Naturally, the same rules apply, that the more consistent this reg, the better.