It's not good to mix RAM because many times it will not function at all or without issues. Name brand RAM is usually only marginally more expensive than no name RAM. Brand name RAM typically has a lifetime warranty and online forum support. For a dual channel mobo two x 2 GB. DIMMs are better than one 4 GB. DIMM because a single DIMM runs in single channel and is slower than dual channel.
1. I would say that RAM is quite significant when it comes to gaming, especially playing games that require high system specifications. Remember certain games like the upcoming battlefield 3 will require that you have a minimum of 4GB of RAM simply to run the program.
2. You should not mix different core speeds, it is recommended that you stick with one core speed, generally the higher one
3. Certain brand names do have their advantages with the reliability and performance, however certain brands are optimized for particular CPUs and motherboards and should be taken into consideration before the brand name
4. a 4GB stick is better than two 2GB sticks, only because it requires less physical space in your computer case. Take note that some motherboards do not support 4GB per slot, and buy accordingly.