1)Is overclock compatibility a necessary on MOBO if i want to overclock my graphic card/CPU?
2)Should i go for MSI GTX770 4GB or MSI GTX770 Lightning 2GB?
For 2) you should go with the 2GB version. Testing has been done and it shows that there is almost no difference between the performance between the 4GB version and the 2GB version. Here's the page: http://alienbabeltech.com/main/gtx-770-4gb-vs-2gb-tested/3/