Depends what you are planning to do with it - just leave it at the speeds you buy it at, or if you plan to do some beefy overclocking.
Start by looking at your motherboard and seeing both what type and rated speed it will support. New motherboards will all be DDR3, while if you are buying RAM for an older motherboard it may be DDR2. Check to see if you need dual or triple channel RAM - basically buy your RAM in packs that match this - so a matching pair for dual channel, and 3 for triple.
The motherboard spec should also state the highest capacity stick each slot will support - not likely to be a limiting factor on a new build, but might be if you are upgrading an old board.
Now for the speeds and timings. Your RAM will run at the slower of either the rating of the RAM or the maximum supported by the motherboard. Put 1600MHz RAM into a slot that only supports up to 1333MHz it will run at 1333. Put a stick of 1333 into a slot that supports 1600 and you will only get 1333. The same applies if you use RAM sticks of different speeds - they will all run at the speed of the slowest stick. Simple buying advice - if you can afford the RAM with the fastest rating your motherboard supports, then buy that, if not go one speed down and it should be cheaper. In real world performance dropping one or two speed ratings isn't going to make a massive impact and suddenly lose you 20fps, but you might lose a couple. It's really there for enthusiasts who want to overclock and get every last drop out of their system.
RAM timings are a balance between performance and stability. Providing the system is stable then tighter (lower numbers) timings should mean better performance as it takes less time between operations, and so could do more every second. However it become a bit of a dark art if you want to go tweaking the timings. To some extent they could be seen as a quality indicator on the sticks you are buying. Generally all the sticks will have been built from the same chips, but some are a better quality than others. The parts that are capable of running at better speeds can be sold at a higher price, so are branded up with tighter timings or higher frequencies, when in reality the same chip of a lower quality will be on another stick, just branded at a worse rating.
Finally a legacy issue, but if you are still running a 32-bit OS it will only be able to recognise a maximum of 4GB (including graphics memory). If you have a 64-bit OS then you shouldn't be seeing any practical limitation.
A bit long-winded but should cover most issues :)
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