Let me just throw some information in about over-clocking and CPU's.
There's quite a few users I see on this forum who say they wont over-clock the CPU, now for some this is simply because they just don't feel like they need to, or don't want to. However the majority say they wont be over-clocking because they don't actually know how to do it.
If your a user who won't be over-clocking due to lack of knowledge, keep reading
If not just ignore the rest of my post.
Most, P67 and Z68 motherboards (socket 1155) will come with built in, over-clocking software. This require's zero knowledge at all of how to over-clock or anything involved. ASUS especially comes with a great AI Suite that's very easy to use, has a nice simple GUI and a big red button that says "OVER-CLOCK MY CPU". From personal experience of this, my motherboard over-clocked my i5-2500k right up to 4.5GHz (using a budget after-market cooler). That showed almost 35% performance increase on the stock speeds. Even with the stock cooler your looking at 4GHz or so.
Now lets look at the costs, in the UK at least:
i5-2500 - £162
i5-2500k - £174
For £12 difference, is it really worth missing out on an opportunity for 30%+ extra juice from the same CPU?
The only reason I can see to justify not wanting to spend that little extra would be if you planned to use a H61 motherboard that's really budget based. H61 boards can go for as little as £45, where-as Z68 boards are closer to £75 for a budget one.
Bottom line really is, don't dismiss over-clocking purely based on that your unaware of how to go about doing it. There's plenty of walk-through guides showing how to over-clock i5-2500k's manually, or as written above, use the motherboards features.
Just my thoughts anyway