The best $300 card available is the Radeon 5850. http://www.newegg.com/Product/Product.aspx?Item=N82E16814102884
Sapphire is also a great brand.
You could also go for 2x5770 which would be a little bit more powerful, but I don't know how crossfire + linux works out:-)
The 4870 is more expensive, and not worth it compared to the 5770 now, and the 4890 is $210, which is a solid deal, but a 5850 is a better card, has DX11 capability and can run eyefinity. This is not to mention the fact that both the 4870 and 4890 are getting harder and harder to find.