I have tried both SATA II & III SSD's on this G74 and you will not notice any speed difference so save some cash and go with a nice SATA II. Unless you just have to have some bragging rights. LOL
Chastity@ASUS has previously mentioned disabling the C States with ThrottleStop to obtain better SSD SATA3 performance. Don't know if this actually works, but you could give it a try. Be aware that ThrottleStop needs to be running for this configuration change. It cannot permanently disable the C States. However it is possible to configure it to launch in a minimized state on Startup.