Your PSU is good enough to handle something like radeon 7850. I'm guessing your reason for not getting that is due to your budget. You've never said what game you wanted to play.
The 6670 should be able to handle all games up to BF3 on Medium High at 1080p so you should be able to run it easily on high/ultra at 1366x768 at a more than adequate frame rate. I would highly recommend you to upgrade the ram to 4gb or 8gb if you can. 6670 should be able to fulfill all your needs but if you ever wish to do something even more intensive, I'd recommend you get a better GFX card just in case.
Here's what I would recommend:http://www.newegg.ca/Product/Product.aspx?Item=N82E16814202011
For your resolution, you should be able to max out just about any game. The card also comes at a great price at just $120. Also, if you live in Canada you can get Far Cry and Blood Dragon along with it! (Newegg.ca)
For RAM, you can choose just about any 4gb/8gb ddr3 1600, just pay attention to ratings and reviews and people's experiences.