I still don't understand how people can get through so much RAM, I'm still on 2GB, although that's on a heavily optimized rig I only use for gaming, so I don't do any multitasking.
Though at work I have a double-height taskbar full to the brim with running programs and never use more than 1.5GB. Hey ho.
The main difference between brands is their warranty. Crucial memory comes with a very good warranty, as does Corsair. Although I have had a faulty stick of Corsair and so has a friend, so your millage may vary. What sort of stuff do you intend to do with your rig? I'd hazard a guess at Photoshop / Modeling?
Get some 1333MHz RAM or some 1600MHz. What CPU do you have? If your FSB on the CPU can't keep up with the memory then there's no point splashing out on anything fast unless you upgrade your CPU as well.
There are subtleties in RAM of the same frequencies, or example they might have different timings, or use buffering etc. Unless you plan on overclocking your RAM (don't) then this is of no concern to you, so whatever one you pick doesn't really matter. If the work you do is critical then I would recommend getting some buffered ECC RAM, though this will come at a premium.
Don't be fooled for buying low-spec DDR3 that are more expensive than others just because they have heatspreaders on, they're just a marketing ploy. In fact I've seen some where the heatspreaders were made from plastic. Hmm. You don't need heatspreaders on your RAM unless you're really pushing it.
