What are your intended uses for the PC?
The GTX460 seems like a relatively feeble component compared to the rest of the system. I'd suggest a GTX680 or Radeon HD 7970.
As for RAM, I never rely on the XMP settings. I set the frequency and timings manually in the BIOS (UEFI, more recently) to the manufacturer's specs. That may be necessary to get stable operation with 4 DIMMs. (Or not.)
If you wish to play it safe, use RAM from the Asus QVL (qualified vendors list). You will find a limited selection of kits based on 8GB DIMMS, but I spotted at least one (Corsair, in the DDR3 1866 page). ASUS - - ASUS SABERTOOTH Z77