I love sm625's sig too; for a long-time the 2600K has been the perfect pill for all your ills, but you can't argue with a gem. It'll go down as one of the best chips of all-time because of its price. If it had made its debut at $500, a whole different story would have played out.
Back to the OP, lets be honest, $100-120 for a LGA775 drop-in replacement isn't a good investment now. I've been wanting to upgrade several of my older Dell Core2Duos with a quad, but people are asking $120 for Q6600 and $150+ for QX6700s; heck if I'll pay that when a little more than double that investment gets me a whole new Sandy Bridge or AMD X6 setup with the current chipset, bus speeds, memory bandwidth and IPC. The Q6600 wont benefit you, coming from the Core 2 version. Don't upgrade unless you'll see an IPC increase.
Unfortunately, it only makes sense in special cases, for example, a business with a lot of Core 2 boxes that are multi-threaded CPU bound, $125 for a drop in CPU replacement saves time and money, but for anyone else, the new platform deals are too cheap. If you can't afford a 2500K setup now, save your $$ until you can.
A lot of Kentsfield, Bloomfield and Gulftown owners are going to be very disappointed at what they get in resale a year from now, once Sandy Bridge has saturated so much of the market with solutions for < $300. I see tons of chips going unsold on eBay just because the owners are out of touch with the situation. Heck, I'd be upset too if I had paid $600-1000 on a top-end CPU in 2009, then saw Sandy Bridge come along at a fraction of that.