Leave the cpu alone.
Pick one.......goes as high up the list as the budget allows.
Be aware that some of those cards need a serious psu to support them.
Gpu's in parallel?
Possible but might turn into a nightmare.
Best to stay with a single card.
How high can i go before i run into hardware compatibility isssues? also if i buy another 6770 will it work if i just plug it in to the 2nd slot? Why does my motherboard dxdiag say to be filled by OEM
im looking to spend about 400 on new gear