Ram is generally put in matched pairs to improve performance.
So say you want 2GB total, you get 2x1GB sticks.
This usually gives multiples of 2.
People also tend to get ram that is a perportion of their total ram, so if they decide to upgrade one day, they don't have to chuck out the old stuff.
For example, my board has a max of 8GB spread over 4 slots, so when I got my first ram, I got 2x2GB, ready to expand.
If I'd got 3GB (a 1Gb and a 2GB), I would have to replace the 1GB with another 2GB, wheras now I only have to put more in.
6GB configurations do exist, but they are usually part of 6-slot configurations, such as 3x2Gb to fill 1/2 the slots, which are less common.
It all depends on the architecture of the CPU/Motherboard in use.
Whether; Single-Channel, Dual-Channel, or Triple-Channel is in use.
Almost all processors and motherboards are either Dual-Channel or Triple-Channel, given that Single-Channel is practically obsolete.
Core i7 are currently the only processors using Triple-Channel configurations as far as I'm aware.