I actually think there is some incorrect information being provided here Moh.
RAM is safe so long as the voltages of the chips are the same or very similar... If they are not you could potentially fry the RAM - you may have to adjust it in BIOS. They are correct with the rest though. It will force your ram to the lowest DDR chip you have (so if you have DDR3-1333 and DDR3-1600 the motherboard will choose the DDR3-1333).
However, if the DDR3-1600 requires 1.2V per chip and the DDR3-1333 requires 0.8V per chip you could actually suffer since the 1600 is not getting the proper amount of voltage. You also have to make sure that the chips are in corresponding slots - if you see two different colors for the RAM slots on your motherboard then do NOT put the same chips next to each other. Match the same chips to the corresponding colors - the reason they are colored that way is because they are corresponding DDR channels (you will be less likely to have issues in the long run).
As for GPUs you can actually use two different cards on the same motherboard... The only thing about that is that you won't SLI/Crossfire the cards. Keep in mind the main reason you may want to do this is if you have a second monitor and want to use that for other purposes while gaming.
The other thing you could also do with the GPUs - they must be from the same GPU designer (either both NVIDIA or both AMD) - is offload the second, less powerful card as a dedicated PhysX card. This -may- increase your performance in game on the main monitor (if you are using only one monitor). However, I might recommend having the cards very similar to each other... as in within 1 or 2 years from each other - any further you may experience bottlenecking. I do say it may for a reason as well... The simplest answer is that it's worth a try, and if it doesn't work just remove it!
Keep in mind with more items in the computer, there will be an increase in heat that's generated