Just Installed W3680
Picked up a Xeon W3680 from Craigslist this past weekend for the excellent price of $335 - the seller checked out as a very reliable person.
Blown away by the performance over the stock 4 core 2.66. Synthetic CPU benchmarks (xbench, novabenck, geekbench, etc) all increased by a large percentage (~50-80%). Real world performance/feel is also significantly better. An added surprise is that my CPU temps -- especially under load -- are as cool or colder than with the old processor. I tested with a 1hr handbrake encode and processor temps were about 1-3 degrees cooler (Celsius) with the new processor. I did not monitor fan speed, but in my quiet "study" I noticed no change in fan noise (on my 4,1 pretty much in-audible).
First time swapping out the CPU on a MP -- very easy! Prepped by watching a couple of how-to-vids, and needed to get a longer hex wrench and good thermal compound. The actual swap took me less than 30 minutes to do.
The only questions this upgrade left me are:
1. How long should I hold on to my my old W3520 processor?
2. Where can I sell off the old one and how much can I expect to get?
I would say the upgrade is well worth it even if you have to pay in the $650 range for the CPU.