I usually play on 1920*1080 or 1600*900 Then what should I pick?
Both cards are similar. The 660 cards are bottlenecked more by Anti-Aliasing at 8x vs 4x rather than simply the resolution.
The HD7870's also come with a Far Cry 3 coupon, but I think the GTX660's have an Assassin's Creed 3 coupon.http://www.tomshardware.com/reviews/geforce-gtx-660-geforce-gtx-650-benchmark,3297-24.html
*Note the average gaming performance has the HD7870 beating the GTX660 but BARELY. Other considerations:
1) Note WHICH GAMES ran better on which card.
2) What FREE GAME do you prefer? Assassin's Creed 3 or Far Cry 3?
Basically you can't go wrong either way.
Choose between these two cards:http://www.newegg.com/Product/Product.aspx?Item=N82E16814125443
I simply can't choose between them. If I were FORCED to pick one (assuming you don't care which GAME you get) then it would be the HD7870.