I just found a Sonata III 500 PDF,
http://www.antec.com/pdf/manuals/SonataIII_manual_EN.pdf
The MSI GTX580 is 268mm - http://www.msi.com/product/vga/N580GTX-Twin-Frozr-II-OC.html#?div=Specification
Now 10.5" is 267mm, so I dont know what to say, if the dimensions are exact then you need a few more mm to stop the card hitting the cage.
http://www.antec.com/pdf/manuals/SonataIII_manual_EN.pdf
Q: What size graphics card can I put into the Sonata III 500?
A: The maximum graphics card length is 10.5 inches.
The MSI GTX580 is 268mm - http://www.msi.com/product/vga/N580GTX-Twin-Frozr-II-OC.html#?div=Specification
Now 10.5" is 267mm, so I dont know what to say, if the dimensions are exact then you need a few more mm to stop the card hitting the cage.