Like most things in life, you get what you pay for, the difference between the two systems is that with the Camsoft package, all the motion control is done on a motion control card within the PC that acts to some degree separate from the PC and handles the motion control independantly by inputing encoders and outputting amplifier signals to close the loop. Other systems that are cheaper use the parallel port of the PC to output step and direction signals to individual amplifiers. There is a learing curve to all the above, if you have not been exposed to CNC architecture before.
Also, incidentally, you should check to see if you Fanuc motors have encoder or resolver feedback, as resolver use was common on that vintage, if so you will need to upgrade to encoders to use the existing DC motors.
The motion card that Camsoft use are Galil and can be found readily available on ebay for good prices e.g.http://cgi.ebay.com/ws/eBayISAPI.dll...873612037&rd=1
BTW if you want to get a feel for what it takes to tune a system etc, you can see 'Live' lectures and demonstrations by the originator of Galil, Dr Jacob Tal on the Galil web site. http://www.galilmc.com/