You don't say which marker you are planning on using it with, but I'd assume it's something like a Piranha or a Spyder. For these markers the PMI regs are good value for money (obviously if you spend more you can get a better regulator - the Palmer Stabiliser is one of the best regulators you can get for CO2 but it costs a lot more.) The PMI reg needs to be kept well oiled (a few drops in the bottom line before screwing the tank in does the job) as it tends to leak if it is allowed to run dry.
Any regulator is definately a good investment, as it will grealy improve the consistency of your marker. You should definately get an anti-siphon tube fitted to your CO2 tank, though, as regulators don't like getting liquid CO2 in their internals. An anti-siphon tube will help ensure that only CO2 gas comes out of the bottle. When set up right in comination with the anti-siphon, a regulator will prevent velocity spikes and dropoff during rapid fire. Depending on the other parts in your marker, it may also work quieter and with a little less kick due to the lowere operating pressure.
