Hi fahad619, here's what i think:
Q1:
There are two things that can happen depending on the specific RAM modules (brand, timing and motherboard compatibility),
1. The two modules will work effectively at the lower speed
this is safe and will only make you lose the speed edge of the faster module
2. The modules will not be acceptable by your board together and it will declare memory failure. As already mentioned BSODs
Q2:
For frequency i know that, if one of your RAM is higher in frequency than the other, then higher frequency RAM would automatically down-clock to the lower frequency RAM (1333mhz).
Also, when i look at the specs of your Motherboard I see, it has in dual channel architecture. It wont run in dual channel using three sticks of ram. You need two sticks or four sticks!
GA-B75-D3V (rev. 1.0)
...
Memory
- 4 x 1.5V DDR3 DIMM sockets supporting up to 32 GB of system memory
* Due to Windows 32-bit operating system limitation, when more than 4 GB of physical memory is installed, the actual memory size displayed will be less than 4 GB.
- Dual channel memory architecture
- Support for DDR3 2200(OC)/1600/1333/1066 MHz memory modules
* To support DDR3 1600 MHz, you must install an Intel 22nm CPU.
- Support for non-ECC memory modules
- Support for Extreme Memory Profile (XMP) memory modules
* To support XMP memory, you must install an Intel 22nm CPU.
(Please refer "Memory Support List" for more information.)
I would recommend you to use 4x4 GB or 2x8 GB memory
of the same type and brand. Nowadays RAM is very low-priced!
Regards,
jroot