Hi Bob
I have the Digitrax Zephyr DCC system, and really like it!
It can run up to ten throttles (addresses), which is enough capabilty for my small (4X8' now, but will be expanding in a 11X13' room) layout. That's ten locos, but if you have decoders with advanced consisting, you can run more locos at one time than that. MUing locos together in a single train is easy! The Z has a built in throttle, and if you have a DC power pack or two you can use them as add-on throttles ("jump ports"), too. Of course, you can add and use any other Digitrax throttle you wish to use thru the loconet system. I currently use the Z's throttle to control my decodered locos along with an old Athearn power pack as a jump port throttle to control my one undecodered train (on address "0").
In addition to the locos, the Zephyr can also control other stationary decoders (for turnouts, etc), but I haven't gone there - yet! This is in addition to the ten loco addresses.
One important consideration is the number of functions a DCC system has. These funcitons control such things as various loco lights (main headlights, ditch lights, marker (class) lights, etc. and sounds (horn, bell, etc.). The Z has 9 functions to run these "goodies".
Another important consideration is the ability to write to and READ the CV values in a decoder. Some systems can not read a decoder's CV values, but the Z can and it is a BIG help! You change CV values to do things like change a locos address (to it's road number, perhaps), adjust loco speed (create speed tables) to match different brands of locos you want to MU together, change the light or sound function buttons, etc., etc. It is wonderful to be able to read the decoder to see if your CV change really "took"!
Besides the Zephyr, the Atlas Commander is another fine starting system. Do look at the comparison chart on Tony's site that the other guys linked you to, and ask questions - you've got a good pool of information here!
DCC is a lot of fun, and much easier to use than it sounds! Do your homework, get the system that's best for you, and jump in - the waters fine!