Sandybridge, Ivybridge and haswell all are much easier to over clock than the first gen i7 chips in the 9xx series. Additionally, new motherboards have turbo boost or easy OC functions that make overclocking a) very easy b) nearly impossible to screw up. I'd go with the asus line, as they have a number of failsafes built in. In terms of pre-built machines, it all depends on what you're willing to spend and who you buy it from. I'd go with a small builder for savings. but you will get less tech support, obviously.
The advantage of haswell is less heat output and less energy consumed. Great for mobile devices and laptops but not many advantages in a workstation where psu isn't an issue.
IMO the best chip out there is still the sandybridge core i7 3930K, because it has six cores, overclocks VERY well, and generates much less heat than the ivy bridge chips. My ivy bridge machine runs almost 20c hotter than the six core for whatever reason. So if you can find or build a machine with the asus p9 motherboard and a core i7 3930K for a little more money, I'd say it's worth it.
