Why would it be a waste? High-end Sandy Bridge CPUs are still formidable for gaming, especially when overclocked.
Socket 1156 is Lynnfield (Nehalem), not SB.
Besides, buying expensive new parts to fix up an old machine is always at best a questionable proposition. You'll be
spending a lot of money on something that
right away, still boxed has a value of less than half what you've paid, that you'll only use for a short while, and that won't be worth anything once you're done with it.
Simply put, $150 spent on a mobo for a 4 year old CPU is $135 down the drain for 2-3 years of use. $150 spent on a new mobo is $80 for 2 years of use or $135 for 6 years of use. $400 spent on a new mobo+CPU combo is $150 for 2 years of use (used CPU sell well) minus the residual value you'll get out of your CPU, which, again, probably doesn't suck all that badly if it's a good one.
In the end, selling your old CPU, buying a new CPU+mobo combo and selling it 2-3 years later will cost you only a little more than buying a new mobo for your old CPU and selling it+CPU 2-3 years later. All the while you'll be enjoying a better system.
Buying a
used mobo for a used rig is a much more sensible value proposition, of course.