Upgrading HD 4850

Hello,

I'm looking for a replacement for my old card.
It's for gaming mostly and I like to crank up the detail everywhere so the one I have just doesn't do well enough for me with the resolution I use.

I don't have much experience with Nvidia cards and I've thought about the HD 5850. Never had a crossfire setup so far but it's not out of the question, as long as I can keep the cost somewhere around 300-400. Must has excellent performance  

I am running:
Antec 900
Asus P6T
Sapphire HD 4850 1GB
Corsair HX620W
i7 920 @ 3.36GHz
Asus VK266H 25.5" @ 1920x1200

Should I maybe also have a better PSU?