It's been a while since I've been CPU shopping, though, so I'm not exactly keen on what I should be looking for in the newer models. Things have changed a lot since the old Core 2's.
I'll make it easy for you as I run two identical sandy bridge machines at home to do extensive video rendering, although I use one as a gaming machine downstairs as well. I just built the second one a few days ago and made it identical spec to my first as I was so happy with it:
Intel 2600k cpu, $314
Asus p8p67 pro mobo 1502 bios, $179
8gb Corsair Vengeance 1.5v 1600mhz ram (black casing), $99
Cooler Master V8 cpu cooler, $50
Use your old case, video card, hdd, etc with that. In the bios chose advanced and set the clock multiplier at 43 (4.3ghz) and downclock the ram a bit to 1333Mhz speed. So for $642 + tax you now have a screaming fast machine that will run at 4.3ghz 24/7. Mine render pretty much non stop and the cpu runs at ~60 Celsius at full load, and voltage is at a low 1.28. I have run these machines at 4.8ghz as well and they were fine but temps and voltages really climbed, so I've left them at 4.3ghz. Note, it's very important to use 1.5v ram! I had issues with 1.65v ram on these machines.
For a speed reference I did a video encode test, my old stock i7-920 took ~18 minutes to do the encode, this new sandy bridge machine took ~6 minutes. I've been using both of my sandy bridge machines non stop and they have been rock solid, they are identical save for my office version having 16gb of ram. Very happy with these machines,