I built a machine that has:
- i7-960 at 3.2 Ghz
- 8GB of DDR3 1600 ram
- average, not frills hard drives (one 500GB for programs/OS and a 2TB RAID 1 for file storage)
- GTX 480 graphics (total overkill, but the team gave it to me…)
- 1000W power supply
I will honestly say I have not really used CAD software on it, but I have streamed video and played online games at the same time with no problems at all. Honestly, I would bet that any quad core processor over 3Ghz will serve you fine if the rest of the machine is built roughly to the same standards (read: don’t skimp on the rest of the components a good processor will be key). Also, prices have dropped so much it gets easier every week to build a machine.