Xeons are no better than mainstream CPUs for CAD. And much, much more expensive, even if you buy the obsolete versions you have quoted (with triple-channel RAM). The latest 6-core i7 CPUs can run a single core at up to 3.9Ghz. Most of them overclock to almost 5Ghz. 1.6Ghz? Look at this:
http://www.cpubenchmark.net/cpu.php?cpu=[Dual+CPU]+Intel+Xeon+E5603+%40+1.60GHz
Even the AMDs are faster!
Dual Xeons would be twice as pointless.
Dual Quadros is a waste of time. SLI/crossfire doesn't work for Quadros.
The rest of your PC spec looks about 5 years out of date. 300GB Velociraptor? 2Gb memory modules?
You could get a way better machine and get your whole office drunk on the money left over.
Don't buy a laptop either. The spec looks a bit more up to date, so why can't you get a proper computer with the same spec?