Normally, a 8 pin motherboard socket is used with high powered cpu's that will be overclockes. Normally populating only 4 pins is sufficient. Verify that you have your 4 pin connector installes in the proper side of the socket. It should be keyed to only fit one way.
If in doubt, you can buy a molex to 8 pin adapter cable that will do the job.
1600 ram is really 1333 ram that can be overclocked.
It should default to 1333.
look at the lable sticker on the mb (serial number) look to see what rev the mb bios is at. then look to see if your mb can post with that cpu and combo. most of the older mb can post with ivy cpu you just need the 13xx or higher bios. if that the issue you can rma the mb to asus for flash rom upgrade or see if someone has an older sb cpu you can use. per asus and it happens a lot with some power supply the 8 pin connectors dont fit on some of the asus mb. but it ok to use 4 pin in the 8 pin connector.