Just wanted to chip in to say that I have a Q6600 and I was just about to change it for a i5 2500k or i7 2600k for about £1,000 (almost completely new system build).
However, I just sat myself down, looked at the way I used my PC and really thought: do I really need it?
I browse the web, do a bit of work, convert the odd video to my HTC phone and enjoy gaming - but largely seem to play slightly older games as I cannot stand paying through the nose for new games so my current setup is fine.
Last month I upgraded from a 512mb HD4870 to 2 x 5850 Extremes in Crossfire (only £200 for the pair new, got £45 for my old card on Ebay, so only really cost me £155) and I am now maxing out any game with comfort (not saying I could do this with all games, but just the ones I currently play). Sure, my Q6600 is too slow to drive the graphics cards to the max, but games still hold above 50 fps at maximum settings, so why should I care that a i5 2500K would more than double that rate?
After deciding to not buy a whole new system I have just ordered an Corsair AX850 PSU to tidy up my existing build - which is a massive bundle of wires - to the extent that I cannot add a new HD without unplugging everything and I have also ordered an Corsair Force 3 120gb SSD - as people seem to suggest that these have a massive impact on everyday performance.
So I should now have a great PC with some components (graphics cards, PSU and SSD drive) that will not only move to a new PC, but will be even better in a new PC when the time comes.
So, I'll wait for Ivybridge I imagine because that could be a significant gain in not just performance, but also energy efficiency as well.
Until then, my Q6600 (admittedly overclocked on liquid cooling to 3.5ghz) will be fine for everything that I do.
I can also watch Q9550 / Q9650 / QX9650 on Ebay to see if one ever goes cheap, or if my Q6600 ever decides to fail on me.
I guess what I'm saying is: tinkering with PCs is fun, but very expensive, so if you can stick with a socket 775 to do what you want it to do, then go for it because you can always add top quality components gradually until the time comes when no socket 775 processor can provide you with the performance you can accept anymore.