I myself usually go with nvidia, but in this case the amd cards are better when compared to the 550. There is a gtx 560 SE which is similar to a normal 560 but it is a bit lower performing, but it costs less than an ordinary 560 too, and performs better than a 550ti. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
It costs $129.99
I have heard Nvidia has a large stock of GTX 560 Ti and GTX 570s they are looking to get rid of before they release the GTX 660. Who knows prices may come down even more. At $129 this card is around $100 cheaper than I saw any 560 ti cards on newegg yesterday.