I don't recommend Asus mobos due to them being RAM sensitive and unreliable IME of building high end PC for the past 20 years. I'd recommend Gigabyte mobos for performance and reliability. The AM3+ is the way to go as it will run the Phenom II CPUs, the FX series and perhaps even the BD upgraded Piledriver core CPUs.
The 900 series chipset mobos are what you want. If you are not going with SLI/Crossfire then the 990XA-UD3 mobo is a great mobo at a reasonable price. The 970 series is also good if you're on a tight budget.