Incoming shameless plug... My Galaxy GTX 560 is for sale on ebay for just $89 right now with a day to go. I won't post a link, that seems just too shady. The 560 is a good card, I just bought a matching pair of MSI gtx560 for sli and so I'm selling of the galaxy I just bought in February.
Both a GTX 560 and a HD 6850 are good cards (comparable performance), the 6850 draws a little less power and that could help if you have a smallish power supply and try going to crossfire/sli later on. However, I hear of a lot of driver problems with the ATI cards compared to Nvidia.
Newegg links:HD 6850GTX 560
wow, prices are going up, I didn't pay this much...
These cards are both good at running almost all games on one monitor at High to Ultra at 1910x1080 resolution. Some games like Metro or Crysis need to be scaled back a little, but still look great. I don't have WoW or Diablo to tell you how they run though.
If you want a multi-monitor setup then you will want better or more cards. If you don't play at 1080, well you should because it looks so good.
i5-3570K @ 4.6GHz l Asus P8Z77-V LK l 8gb DDR3-1600
MSI GTX970 l Crucial 500gb SSD l 2tb WD black HDD + 1tb Barracuda
ThermalTake TP1050 l NZXT Razer S340 l Qnix 2560x1440