A lot is determined by the games you will play.
For graphics bound games, or triple monitors, the sky is the limit, say a GTX690.
The stronger the card, the better eye candy, and consistency of response times.
If budget is not an issue, a GTX680 and a quality 650w psu.
You can always move it to a later cpu upgrade.
More realistically, I would agree with a GTX670 or GTX660ti and a quality 550w psu.
Im gonna be mostly playing Black ops 2 Borderlands 2 and far cry 3 on a 24in asus 1080p monitor..i found a ASUS GTX660 2gb version for 239$..do you think that would run everything on high?