I dont plan to crossfire right away. My thoughts were to go with the cheaper phenom until Piledriver comes out and is affordable. I just dont want to get the phenom if it will be trash in 6 months. The FX-4170 is newer but a lot of people seem to like the phenom. I plan on playing a lot of FPS and MMOs if that makes a difference. This is my first gaming rig. I want to be able to play games on a decent setting for a while.
Even though AMD released the good piledriver cpu's, posters still keep trying to move people to Intel. If the OP said he wants to build a AMD machine for gaming then answer that question and yes they are AMD chips that are good for gaming. Get a FX 4300 or 6300 instead of the other two you asked about. They are not priced much higher.
Your best invest will be faster processor and single higher end GPU.
You are paring a couple of HD7850 *1GB* version, those wold be bottlenecked by the Vram. And also not all games will be able to run Xfire properly or scales well. Sometimes Xfire comes with anoying problems and stuff, other times Xfire is just plug & play.
For 330$ you can get a really good HD7950, that overclocks beastly and may get the performance of a vanilla HD7970, which is plenty for todays gaming and a single high end GPU comes with less headaches than a Xfire solution.
Also, the cheapest HD7950 3GB that you can find will be around 300 bucks thats 30 bucks left to get a better processor. As you are getting an aftermarket cooler, you should get an Piledriver Chip aswell. Best recomendation for that build.
FX 8320 + HD7950 or
FX 6300 + HD7850 *2GB version* + SSD 128GB
I mean, with that money you can get a nice machine that will be more than enough for todays gaming !