Size Matters - 580 in Antec 300

if i'm not mistaken 5850 have the same length as 580. so far nvidia longest card are 10.5 inches

Maximum video card size: 11"/279mm (Note: some video cards may block access to the adjacent hard drive bay)

http://www.antec.com/Believe_it/product.php?id=MjQ=

looks like you are good to go

Thanks  

My GTX480, same size as the GTX580, fits nicely in my old Antec Sonata case. You have to kind of turn it sideways at first, but once it's in, there is plenty of clearance around on all sides.