so my mother board requires 8 pin 12v. but my psu only has one 4 pin 12v. can I use an adapter from my 12v molex rail? not gonna have anything else on that 12v rail anyways
Yes and they do make adapters. You can usually find them on Amazon for super cheap. I do however have to say use caution because if the power supply does not support 8 pin by default then it may not handle the load if you are overclocking. Non-OC'd should be fine though. I also wouldn't load down the PSU with a big beefy GPU. Once again it all depends on your total power needs and what the PSU can handle.
http://amzn.com/B002O21XHQ for the adapter.