I have a 27" iMac (late 2009) with an ATI Radeon HD 4850 512 MB graphics card.
What are my options to upgrade the video?
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Late 2009 and newer iMacs have got a removable GPU so you can replace it, although Apple does not allow you to do it and it will void warranty if your Mac is in warranty.
If you still want to replace the GPU, there are guides to install a newer GPU on your iMac > http://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+or+2374+Graphics+Card+Re placement/9553
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