Out of curiosity - what is your goal in moving to a more power efficient system? To save money? Upgrade to a more powerful processor? Have a little fun building a new system? Or all of the above?
If it is with the goal of saving money, you might want to crunch some numbers first to see if your plan makes sense. I'm also running a Phenom II (x3 710) - with the complete system pulling about 65 watts from the wall. When I read the reviews for the new i3-2100, I thought it looked like a great solution to replace my CPU/Motherboard with something more power efficient.
Here's the thing though - at 65 watts, and about $0.12 per KWh, it would only cost me about $70 a year to run this machine 24 hours a day. And I don't even run it 24 hours a day - it is sleeping half the time - so it's more like $35 a year in electricity costs.
Meanwhile, to buy a new i3-2100 and H61 motherboard is going to cost at least $140 (current price at Microcenter). I would also need to upgrade to DDR3 memory (currently using DDR2) for about $60.
So assuming that my i3-2100 system draws half as much power (say, 30 watts), I'm looking at saving about $20 a year in electricity costs with the i3 over the Phenom II (assuming it sleeps half the day, or $40 a year if I were to run the system 24/7). At that rate, it would take 10 years to make up the price difference
, by which point I will have long since upgraded to something else. And it would take considerably longer to make up the cost on the i5-2500k system you are looking at.
From what I've seen, it just isn't economical to throw out (how much do you think you can sell yours for?) a working CPU with something that draws less power in the hopes of saving money. Of course, with the i5-2500K you will also be getting a pretty big performance boost, so that might be a better reason to look into upgrading. But I personally haven't run into any situations yet with my Phenom II where I thought, "Damn, I need a faster CPU".
So that brings me to the last reason possible reason for upgrading to Sandy Bridge: boredom. One of these days I'll probably be bored one weekend and find myself driving down to Microcenter in search of a new weekend project. And if that's where you're coming from, I totally feel your pain.