Yes, and NO.
Yes, if you had other brand MB, may try change the BCLK to oc, which is similar to this: http://www.tomshardware.com/reviews/xeon-e5620-overclocking-westmere-ep,2767-2.html
No, because you had the HP OEM PC, you can't do that, they locked that feature. Sorry.
You are partially correct. My HP Z400 (C602 chipset) is able to push my W3680 to 4.1GHz on all cores with ThrottleStop.
The W3680 has an unlocked multiplier. He won't be able to do much of anything with his E5-2643.