The more RAM the better!
If you can afford 16gb, get it.
FCP struggles with AVCHD, many people use a conversion utility called Clipwrap. This does a great job of sorting AVCHD files into a format that FCP X is happy working with. Steve Douglas did a review of the app here on Wetpixel, and if you search for Clipwrap, there has been loads of discussion about it on the forums too.
Spot on cheers Adam!