For $150 I would suggest going for the 6850.It's faster than both the GTX550ti and 6790 and it O.C.'s nicely.
XFX 6850 $160=$130 after MIR + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814150551
You also get a free game with that card.
If you can stretch your budget just a little more this card is the best bang for your budget.And it even comes to within your budget after the MIR.You won't be disappointed,I know, I own one.
Sapphire 6870 $170=$150 after MIR + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102948