It will all run at the speed of the slowest modules. If it boots with all 4 sticks, you are okay. If it will not, boot with only the 2 slowest modules installed, and enter the BIOS. Turn off the auto detect or auto configuration for your memory, and manually set everything to the speed those 2 sticks work at. Insert the other 2, and give it a try.
Yes, both banks should be running in dual channel as you have them plugged in.