As said if you are looking for long time use i highly suggest the Quad core processor (280$) , the new G0 stepping overclock pretty high.
The Pentium processor will do the hole job fine even @ stock speed . For high resolution the video card will be your first limitation .
For the Exxx overclock not all people can do with their chip over 3 Ghz , many already reported about FSB wall @ 400 mhz . the average overclock is 2.8-3.2 Ghz.
Auto cad just like other application will run pretty fast even @ 2.8 ghz.
For 90% (3.6 GHz) u have to go with :
1) 45 nm processor .
2) Q6600 or E6750 .
It's pretty much confirmed that Exxxx walls at 400 FSB. I had mine over 400 for a while, but strange things started happening even at low temps. I've only got this E2140 running at 3.2 GHz (sob, sob) but if you consider the fact that it was a 100% overclock, the price/performance ratio wasn't bad at all
I wouldn't recommend it now though.