I'll spare you the rant on what I've been through in trying to obtain a great CAD machine for the type of programs you cite in your question - in a nut shell (my experience) the single biggest advantage to a XEON over the newer i5/i7 processors is the memory issue. From Intel "The Intel® Core™ i7 desktop processors and desktop boards typically do not support ECC memory. ECC memory is usually used on servers and workstations, rather than on desktop platforms." That said, there can be a motherboard here and there that claims to support it and I think there are actually 3 of the higher priced i7s that support it.
If you do a price compare you will find that the new E3 Xeons are a good buy compared to the i7's - i7-2600K Sandy Bridge 3.4GHz (3.8GHz Turbo Boost) LGA 1155 95W Quad is about $300 and you can spend almost anything you want on memory - but it appears to me that the single core number is what you need to pay attention to, not so much how many cores. - with all but the really complex models, hyperthreading take off a good amount of the load and it works with a good graphics card.
The Xeon E3-1270 Sandy Bridge 3.4GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1155 80W Quad-Core is about the same price - but - it fully supports ECC and from experience, this is a big difference in the amount of stability you get in the system. So I would always opt for stability - but both will perform almost identically in terms of actual system speed - assuming all things are equal. And ECC memory direct from say Crucial is a good buy - cheaper than the likes of Newegg, etc.
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