Not enough information for any kind of recommendation. We don't know what those users do or how the solution is designed. Go by what your current FMS stats log say and build off that.
Some more questions to prompt thoughts:
- webdirect in use?
- do you use a lot of PSoS?
- any growth in the solution over the next 2-3 years (# of records, # of users, new modules,...)
Go for the fastest disk i/o that you can get
Go for a decent amount of RAM (32GB or so)
The hardest choice is in the usual trade-off between the # of physical processors, the # of cores on each and the speed of the cores. That's where the nature of the solution and your design comes into play.
wimdecorte has a point. Are you having any issues with your Mac Pro that are leading you to look at new hardware or are you just looking to update the hardware because you can?
I will agree with wimdecorte that the disk i/o is paramount. If you do not already have some sort of SSD RAID storage I would suggest you do that. That has been the single biggest performance improvement for the money on most any deployment.
The other thing that is often overlooked is the network connection to FMS. Increasing bandwidth for the remote clients when they are connected and the actual FMS itself is a good place to look for remote connection improvement. Upload and download speeds at both ends matter.
The rest of the server hardware really depends on your solution.