I run 5 using a 950 OC'd to 4GHz and 24GB of ram. I'm also using 2 MSI GTX 560 Ti's OC'd to 950/1900/2100. I use 2 1920x1200 monitors and run the game in FS at 1920x1200.

I play on the low setting and get 40-45ish fps on the main screen.

IMO, the biggest performance bottleneck in Rift is CPU, then RAM size, then GPU.