I would suggest going with the 6850.It's faster than the GTX550ti and a little faster than the GTX460. It's also light on power. And price is good for the performance it provides.For future reference 6850 crossfire scales very good.
What are your full system specs?(CPU,RAM,etc.)
What is the make/model of your PSU?
Sapphire 6850 $150=$135 after MIR + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814102908
That card is actually on sale so it's only available at that price for the weekend only.