The reason I went with my Corsair Hydro 110 was that it's the largest pre-packaged cooling setup available. All the reviews were saying that that's the size where they started to see liquid cooling pull away from their top performing Air cooler (the notcturna that Kludger mentioned). Even then, it's performance just barely scrapes ahead of the nocturna. Plus, not all cases are drilled to except rads that big. (This means I "had" to get my awesome new Corsair C70 case, what a shame :D)
That said, Liquid cooling is popular because a smaller, lighter weight* cooler can do the work of a much, much larger and heavier air cooled radiator. It just costs more, and does carry the (often exaggerated) risk of leaking. LC setups also carry a shorter shelf life than AC setups. With an AC rad, all that can really fail is the fans, and those are cheap and easy to replace. This isn't a big deal to me personally though, as I plan to upgrade my cooling solution with my CPU anyway.
My advice would be to start with a Coolermaster Hyper 212 (link below). I used one on my AMD setup, and frankly you can't beat it without spending 3 times that much. See how much of a boost that gives you, and go from there.http://www.newegg.com/Product/Product.aspx?Item=N82E16835103065
*Important when shipping a pre-made PC. Big, heavy air collers put a lot of stress on the motherboard as that's where they are bolted to. LC setups put the bulk of their weight in the radiator, which can be bolted to the case itself.