System Usage from Most to Least Important: games maxed like: far cry 3, battlefield 3 online part, wow, metro, bioshock, surfing, email
Are you buying a monitor: no
Parts to Upgrade: cpu / mobo / ram/ vid card
Do you need to buy OS: no
Preferred Website(s) for Parts: newegg / tigerdirect
Parts Preferences: intel, nvidia
SLI or Crossfire: no
Your Monitor Resolution: 23” at 1920x1080
Additional Comments: current system : amd 965be at 3.8ghz with 212+ push & pull, 8gb gskill ripjaws ddr3 1600, xfx hd6950 2gb, gigabyte ga-ma785gmt-ud2h mobo, corsair tx750 ps - wd black 1tb hd, haf912 case with 4-120mm fans and 1-200mm top fan
And Most Importantly, Why Are You Upgrading: case is new but system is 2yrs+ old seeing if upgrading is worth it
If I was you I would stick with what you have, but since you are asking about upgrading you must not be happy with current performance. Could you buy a second 6950 for crossfire? That wold be the easiest upgrade. If you want to spend all $1000 then you could get a 3570k/extreme 4 and a 7970 GHz. Your ram/psu/HSF/case and hdd are good. You could also throw in an ssd if you wish.
The system you have isn't terrible, i'd try waiting for Haswell and the latest round of GPU's before upgrading. If things aren't good for you though then you should upgrade. Personal preference really.