Do you mean i7-3770?
1) First, you must find out how much memory your motherboard supports PER SLOT. I could not find a support site for that motherboard, but my Z77 motherboard for my i7-3770K only supports 4GB of DDR3 memory per slot.
I have 4x4GB 2133MHz CAS9 (16GB total)
So unless you know for sure 8GB is supported you should stick with 4GB sticks.
2) This is likely too expensive, but this MIGHT be more ideal for you (4x4GB = 16GB total):
https://pcpartpicker.com/product/VcDwrH/corsair-memory-cmz16gx3m4a1600c9b
3) MORE MEMORY may be more important than having sufficient bandwidth so DUAL CHANNEL may not matter much too you. For example, browsing the web and probably most games may not benefit much above 1333MHz Single Channel. I suspect GTA5 and Witcher 3 would though (also depends on graphics card, resolution etc).
4) MIXING sticks may or may not work in terms of compatibility. I've done it a few times by adjusting the CAS and other timing values to fix issues where MEMTEST86 failed.
So while you SHOULD stick to a single kit designed to work together like a 4x4GB kit (at least $130USD) you may or may not have success buying something like:
a) 1x4GB 1333MHz, and
b) 1x2GB 1333MHz, then
Set the 2x2GB in Channel A (may be for example BLUE), and 2x4GB in Channel B (may be BLACK slots).
That may cost $50 (ish) though you may get lucky with E-Bay too.
5) I believe the basic specs are DDR3, 240-pin, (non-ECC).
If an i7-3770 some programs can benefit up to 2133MHz CAS9 DDR3 setup in Dual Channel, but again most applications aren't that memory bandwidth dependent.
6) ONLY an example, but I looked on EBay and saw this:
https://www.ebay.com/itm/CORSAIR-XMS3-DDR3-1333MHz-16GB-4x4GB-DDR3-DIMM-CMX4GX3M1A1333C9/182887438190?epid=140610328&hash=item2a94f0d76e:g:Y~cAAOSwvD5aA7NW
4x4GB, 1333MHz, DDR3, non-ECC, unbuffered, 240-pin
Again, not 100% sure that's the right memory for your motherboard.