Why are you set on getting a cheap/poor power supply? And a single 280x would be recommended. Definitely wouldn't want to run Crossfire 7850s on a 650w power supply, no matter the quality.
If you already have one of the 7850s then crossfire is a good option. Otherwise I would say stick to the r9 280x, a single gpu is typically more stable and you don't have to worry about support for specific games.
Why are you set on getting a cheap/poor power supply? And a single 280x would be recommended. Definitely wouldn't want to run Crossfire 7850s on a 650w power supply, no matter the quality.
2x 270x run at about 420w from the wall with my cpu so yeah...???
Nope, your power supply is pretty bad. Cooler Master PSUs review badly and are known to fail with alarming regularity. It's not a good idea to Xfire anything on that PSU, no matter how low the wattage is.