it depends on the user. some are fine with c2q even these days.
Ditto. Just trying to keep up with the latest and greatest is an expensive hobby. For me, it's a tradeoff that involves the $$, the security and integrity of my data, and the need for ongoing routine use of the machine -- against that famous line from "Top Gun:" "Do you feel the need for speed?!"
Truth is -- and there's a thread here near the top of the stack per the old Q6600 C2Q -- I'm not even sure that for many purposes those old LGA-775 systems are just ready for the junkpile. I might have waited another year before building this system, in which case I'd be using an Ivy core and a Z77 board.
My fam-damn-ily is using C2D's on LGA-775 mobos -- one with Win 7 and the other (get this!) still using XP 32-bit. I just have to keep my hand in it while I keep up with the web-reading. . . .
Usually, I just build a completely new machine instead of harvesting my used parts -- all for reasons cited. There are certain types of "overhead," for instance if you use the machine for double-duty in HTPC functions and you don't want to mess with the HDCP reset and configuration.
For the 2500K, I think we'll need to wait for this Haswell core release just to "see." Ivy won't make the 2500K obsolete . . .