I want to buy another GB of ram, and I am looking at this one set of 512's by KingMax. Currently, I have 2x512MB of Corsair in my machine, and I want to add this 2x512MB of KingMax into my machine. There speeds differ as well, but they'll be in their respective channel sockets.
Depends on your board, bios, and memory settings. I use 3 different brands of memory, all the same size and speed. To get it stable, I set the timings manually. Even if it works, your memory will default to the lower speed of the 2 pairs.
Possible on many of the new P35 motherboards because they have Intel Flex Memory Technology enabled. All the DS3x series motherboards have this. It enables RAM with different Speed/Size to remain in Dual Channel mode. Look at http://www.intel.com/design/chipsets/express_flyer.htm