I've got a 6950 that i'm trying to get rid of and its a great card! its the 2gb version, runs anything i play at max settings/max res! highly recommended.
I would highly recommend either the ATI 6850 1GB or GTX 460, both are excellent cards and excellent upgrades from a 4850. I have tried both of those cards, and I had a 4870 before, and its a huge improvement. I've got the 6870 2gb now, and if you have the money, its completely worth it!!!
For a twenty seven inch monitor, ATI graphics cards work best. My 6950 2GB that i'm trying to sell at the moment is very good at higher resolutions, and the Eyefinity on the ATI really packs a punch with bigger screens over a Nvidia card. I would highly recommend the new 6950, 6970, or 6990...