Xeon CPUs are not the best choice for a DAW.
You'll see no audio related benefit from using them.
In fact, they're often running at lower clock-speed... meaning you'll pay significantly more AND take a performance hit.
While on the discussion of Mac Pros:
Many folks don't realize this... but the latest generation iMac with Skylake 6700k CPU will *outperform* the $4000 Mac Pro (because it's running Xeon CPU and architecture that's older/slower).
Mac or PC, you don't want to sacrifice clock-speed for more CPU cores.In an ideal situation, you want fast clock speed *and* more CPU cores.
If you have to choose one over the other (and there's a significant difference in clock-speed), go with the higher clock speed.
Adding additional unused RAM will buy no additional performance.
You need enough RAM to run the desired projects... to ensure the system doesn't become RAM-starved and start hitting the VM swapfile in lieu of physical RAM (which kills performance).
If you're in a situation where disk-streaming sample libraries are bogging down the machine:
A single SSD sustains over 500MB/Sec.
If you need higher disk-streaming polyphony... and the library only allows a single location (EWSO), you can put a pair of SSDs in RAID-0. This nets about 1000MB/Sec.
If you need insane disk-streaming polyphony, you can run a PCIe (or M.2 Ultra) SSD drive that uses 4 PCIe lanes and sustains 2500MB/Sec. If you're an extreme composer, you can combine several of these solutions.
A modern well-configured machine can sustain over 4000 stereo disk-streaming voices of polyphony.
If that's not enough... you can run a second "slave" VE Pro machine (dedicated to running sample libraries).
Research... to ensure you're getting what you need.