Steve covered the controller aspect pretty good. From the Drive perspective there are also considerations that can be had.
There are
stand alone drives which take in the mains power and control a motor or two.
Then there are what I would call "bus-able" stand alone drives that can stand alone but can also have their DC busses connected with other drives. The advantage to this is that they can share regen capability or the ability to electrically brake overrunning loads. For instance vertical axes usually go into regen when quickly lowering loads. If you have a three axis system (X, Y, Z) but Y & Z don't have any regen you can handle as much regen as all three drives are capable of for the Z axis.
Each of the above has to have a power conversion section that rectifies the AC to make DC.
There are also systems with a main power supply that does the AC -> DC conversion and can have multiple drives hooked to its DC bus. The power supply is an extra component but then each drive is less expensive because it only need to have the motor driving circuits, not the AC->DC part. Some of these power supplies can actually put regenerated power back onto the mains supply lines and save energy vs. dumping the regen over a resister and making heat.
Recently, there are motors that have the drive built into them. These are generally smaller motors but then you can expand the system without adding anythign to the main electrical enclosure. Dunkermotoren has some small DC servo motors like that
http://www.dunkermotoren.de/default.asp?lang=2
Bosch Rexroth has some AC servos that are a little larger with built in controllers.
http://www.boschrexroth.com/busines...antriebstechnik_en/indradrive_mi_en/index.jsp
There is where you can have a truly expandable system...
<disclaimer> I work for a company that distributes both Dunkermotoren and Bosch Rexroth products</disclaimer>