Wrong title. Should read "Phenom on Rehab..."
Best AM2+ o/cing mobo would be MSI K9A2 Platinum for about $150 US. Shop around.
You can do single, dual, tri & quad CrossFire on 790FX. That's the beauty of it. Simliar to the new 790i with single, dual or tri SLI. But you'll have to make sure the gpu supports the config.
Everything except the psu is fine. I don't know if you can do quad CF with 550W.