The primary reason for buying RAM that is 1600 MHZ rather than getting ram of a lower speed and relying on OCing is because in order to OC to 1600 MHZ you will have to sacrifice some of the 1333's performance capability elsewhere. RAM is measured primarily by its speed and its timing. The timing of the RAM has to do with latency. Without going into any extreme details that you probably don't care about it, simply put, in order to OC 1333 MHZ ram to 1600 MHZ would require a higher timing more than likely. If you buy RAM that has speed of 1600 MHZ and a timing of X, the company guarantees that the timing will not have to be compromised in anyway in order to have the product run at the advertised speed. Increasing the RAM's timing will have a negative effect on latency
Also just another point. Some people will say this doesn't matter but I think it is always better to tread on the side of caution: try and download a copy of the manual for whatever motherboard you plan on acquiring. In the manual, there should be a section that outlines all of the qualified RAM vendors that are deemed compatible with that particular motherboard. It will also tell you if the RAM that you are trying to acquire is capable of being loaded into all, or just some of the DIMM slots on the motherboard. The majority of the time, it shouldn't make a difference what it says, but since there are usually so many different options on the list, to me it doesn't make any sense to go with something that the company does not suggest. For all you know, this could someone void the warranty, should you mention that you were using "incompatible RAM" while receiving any technical advice from the motherboard manufacturer.
As many other people have already stated, OCing is probably more painful than what it is usually worth in terms of performance. Unless you do things slowly and carefully, you can very easily fry your hardware. It can get very frustrating. However, when you do finally get the computer running smoothly, it will feel pretty awesome.