I work at a shop where we run a ton of Xeon(3.0-4gHZ) PCs and the biggest bottleneck at this level of performance tends to be the GPU. I'd stick with an NVIDIA Quadro-type card, not a consumer grade. The difference can be as high as 25%(seat-of-pants measurement) by simply using the proper graphics card. As I understand QPI, it is simply the competitor to AMD hyper-transport so I don't think your choice here will make as large a difference as the GPU you choose. Another good point is to load it up with memory at least 12GB.
I should also note that most high-end CAD packages are still single-threaded in most operations due to the nature of the work, so most multi-core machines allow for multi-tasking, but they don't make a whole lot of difference within the CAD environment.
Some things I have noticed from maintaining 50+ CAD workstations:
- Intel CPUs are much faster in this realm than comparable AMDs
- NVIDIA GPUs handle the work better than comparable AMD GPUs
- Multi-cores are not utilized by many CAD operations due to the linear
nature of the work
- RAM RAM RAM...and then add some more RAM! Large CAD files with complex geometry and CAM operations will bring even the best system to its knees due to RAM restriction