Honestly longevity is nowadays not even considered. At this point its safe to push it to the bleeding edge of it's threshold and as long as you take care of the temps it will last you long after Intel comes out with their tock. If you didn't know ivy bridge is only a small leap in technology with the big one coming next after ivy bridge. So in this case I say push it as hard as you want, I've never even heard of an intel proc frying unless you are trying to set world records, and then when the next intel proc comes into production, buy it.
Also, when you install, i know this is obvious but make sure you dont bend any pins. And take caution when overclocking the ivy. It does NOT OC like sandy. read up before you push it. Anand's got a great article on it, from a few weeks ago if you really want to know how to. In my honest opinion the 2600k is still the best proc ever made.
You'll find out during your testing. It's not the same for every CPU.
If you luck into a 'golden' CPU you might reach 4.4~4.5Ghz on stock, stable under load.
4.2~4.3Ghz seems to be more common though. And some of that might be down to motherboard and OC'ing skill of a person.
We don't know if it have the actual game performance boost to entice gamers to upgrade.
Core2 broke new records for x86 IPC, each Core i3/5/7 generation added yet more incremental improvements to that so there probably is not much more left to gain IPC-wise. Unless applications start making more effective use of multi-threading and heterogeneous computing, we are going to keep butting against the single/dual-core performance brick wall.