RAM helps you overclock higher and can help you get better scores when you run benchmarks. In real-world use though, you can't really notice a difference between one speed of RAM and another. I just upgrade everything in my computer all at once whenever I upgrade. If I did not however, I dont think that I would upgrade the RAM very often though. Not something that really needs to be upgraded unless you are going for a super high overclock or something.
I really dont see a lot of difference in one brand over another other than warranty and quality of the RAM sticks. I buy OCZ because they have a lifetime warranty on them. Corsair and Mushkin are great as well, also Kingston. Just buy whatever is best for you and fits in your case. The timings on the RAM also don't make a whole lot of difference when you are using the computer in real-world situations so you don't need to worry too much about that.