Server or workstation? how can I choose a good system for parallel computing of MATLAB?

I need a high performance system for my MATLAB R2014a calculation. beside number of cores (logical cores) I need fast cores, High RAM (near 64GB) and a fast CUDA core Nvidia graphic card. I found that i can buy Workstations or tower servers. What is different and what do you prefer for this? The system is working nearly 24 hour a day for calculation. for example i found these models:

- HP ProLiant ML350p G8
- HP Z820 Workstation

First one in server and second one is workstation. Can i turn on and using server like a PC or workstation? another question. Why server doesn't have graphic card? Can i add one to servers and if not how can i use that for my process? I found that i can add two CPU's to server. can i add two CPU's to workstation?

Thanks.