I apologize but I believe you may have misinterpreted the question, which was a problem on my part, but I thank you for replying. Basically, I was planning on ordering 2x4 (8gb total ram) for my MacBook Pro, but I also found out that my existing ram, which is 2x2 (4gb total ram), is usable in a PC as well. I was planning on making a computer, and I thought I might as well use this preexisting ram. But I thought it might be better to give both my laptop and the PC equal ram. So would one stick of 2gb ram and one stick of 4gb ram be faster than just keeping it at 2x2 ram (4gb ram total), as I've heard many people say that the computer can only run at the lower speed ram. (which is kinda confusing, and i would also like to understand, so post answers to that as well)
Mac is particularly picky when it comes to Ram. I suggest you stick with a matched pair when it comes to your Mac. If you are thinking about upgrading your Ram in your Mac I also suggest you get either,
Let them scan your machine and they will tell you exactly what your Mac will hold and the price as well.
This is Other World Computing and both of these sites sell the top of the line in Memory when it comes to Mac Computers.
As far as using the Mac Ram in a Pc that is fine if the PC will accept the same Ram as the Mac.
Do not buy cheap ram it will cause more problems than it's worth.
So would one stick of 2gb ram and one stick of 4gb ram be faster than just keeping it at 2x2 ram (4gb ram total), as I've heard many people say that the computer can only run at the lower speed ram. (which is kinda confusing, and i would also like to understand, so post answers to that as well)
6GB of RAM might be better than 4GB, again, depending on what apps and how many you open. It wouldn't necessarily be faster.
As far as running "at the lower speed ram" is concerned, that only applies when mixing different speeds of RAM. For example, if a computer's memory bus speed is 1333 maximum and a 2GB stick of 1333MHz and a 2GB stick of 1600MHz is used, the computer will run both sticks at the 1333 (slowest speed).
I ran my Mac Mini with a 2GB (stock from Apple) and 4GB (Crucial) stick of RAM of the same speed and never had a problem. It helped with Page Outs. Meaning, my system didn't swap out to the hard disk space when RAM was at a minimum. I pulled the 4GB stick ard put my other Apple stock 2GB stick back in after I upgraded from OS X Mountain Lion to Mavericks. Mavericks, in my opinion, manages memory better and compresses it so that 4GB total "appears" to work more like 6GB.