Buy the 2 and you're good to go with CrossFireX. See post below for details compared to 7850. Some good links in there with reviews and such as well. Personally I would rather spend the extra $30 and opt for a single GTX 660, but thats just me.
http://www.tomshardware.com/answers/id-1745262/whats-7790-cr…
2x HIS Radeon HD 7790 Video Card - 1GB 128-bit GDDR5 - PCI-Express 3.0 - Core Clock: 1000MHz - Memory Clock: 6000 MHz - 2x DVI, HDMI, DisplayPort - CrossFireX Ready - 2 Years Warranty - includes 2x BioShock Infinite Game Codes
It's cheap for two, but I don't need two - anybody want to split?