what is the best card for my pc..

You've got a pretty old PC which barely meets the minimum requirements for many modern games, so I'd be careful about picking the games to run.

BF4, and especially Assassin's Creed 4 won't run well.

About the best you can do is an HD7730 as I don't believe that needs a 6-pin connector.

If you pick some OLDER or NEW but not too demanding games you can have a good experience still, especially if you properly tweak the games.

Games like DeusEx #1 for example come to mind, or Call of Duty #1, possibly Left for Dead 2 etc. Install STEAM and try DEMOS if possible.

I don't know where the HD7730 is currently sold but another option is to buy a different PSU like the quality Antec VP450 for only $40 and get an HD7790:

1) Antec VP450: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371045 ($30 after rebate)

2) HD7790: http://www.newegg.com/Product/Product.aspx?Item=N82E16814121727 ($110 after rebate)

So that's $140 (after the stupid rebate's).