I think overclocking is almost essential in a workstation. Performance is everything when you have expensive employees waiting on the computer. We run AutoCAD and Revit in my little company, and our manager enforces a very tight budget on us. I built our last 3 CAD workstations myself. I used i5-750 processors, 8GB ram, WD velociraptor drives, FireGL video cards, Antec cases, Win7-64bit, and a good Xigmatek CPU cooler. I also added 2 more fans to the 2 that the Antec cases already had. When I built these over a year ago I wasn't the best overclocker but they would run at 3.9 and pass my stress tests. We run them 24/7 and I set them at 3.5GHz for that. We don't have any temperature problems, the CPU's run very cool. For our software the video card isn't that important, the lowest FireGL will run AutoCAD just as fast as the most expensive one. The reason to even buy the FireGL is to get better drivers that will work with the 64bit OS. I've tried with other brands of cards and they weren't reliable. The only problems we have had with any of these machines has been with the cheaper video card when the FireGL that we wanted to buy was not available. I believe we spent about $1200 each on these machines, I spent about 2 hours each at home building and testing them.
At the time that I built these, the i5-750 system offered a $300 savings over an i7-920 system. Today it might be a different story, and with the i7-SB we might run them at 4.0GHz.