you should test it definitely..
but yes, overclocking is really easy once you get over the initial fear of frying something (just don't pump too much voltage)
good then, if your cpu still BSODs on a long run of ibt/prime95, you might have to raise your llc to level 2-4. I had to raise mine to lvl 1, but avoid it if possible, lvl 1 llc can overvolt your CPU
Also, I have a hyper 212 evo, and I reach 4.5 ghz on my 2500k with temps around 65c on full load (prime95/IBT/folding). Ivy bridge runs a little hotter on average though.
There is no straight answer to how good of an overclock you will get (every piece of silicon is slightly different). But as for the cooler, I wouldn't recommend overclocking on the stock cooler, I don't even like running my processor at stock settings on a stock cooler.
Also, piece of advice, make sure your cooler is decent enough to handle the extra heat.
If not, buy a better one. Always remember one main rule: Heat is your worst enemy. Well, besides grizzly bears.
This guide may be useful to you
http://www.guru3d.com/articles_pages/radeon_hd_7870_overclock_guide,2.html
Personally, though I own a 7870, don't assume from the guide that you can necessarily reach an overclock that high with those settings (each chip is different). Also, they raise their...
Why do people buy alienware? It's such a ripoff.
To your question: It's matter of personal opinion I guess. If you are doing heavy workloads with your CPU or have a wide array of video cards that will be easily bottlenecked , id say yes. As for your GPU, it can be, but honestly to me it isn't...