Can I Add More RAM?

I have the G.Skill DDR2 1066 4GB (2x2GB) RAM modules. What I was wondering is whether I can add another 2GB of the same RAM to get it up to 6.

Also, as a side question, am I able to add RAM of a different brand, as long as its still DDR2 1066? (Lets say I find it cheaper to get a different brand of memory, it would still be DDR2 1066, and I suppose I would need it to have the same latency timings as well)
If this is possible, what do I have to ensure is the same throughout my memory.

It is possilbe, but we'd need to know your specs. Also, keep in mind that unless you have a 64 bit OS, you won't see the new memory you'll be adding.

Also, it is advised that you get similar memory (speed, voltage, and timings). That is why it is often easier to get the same kind as you have.