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It would get mighty toasty in there Jimlad!A single slot HD4850 would be too hot to handle for the shuttle.
I think that a GT220 would be an excellent choice.Single slot,short and takes less than 60W underload.So this would be excellent for your shuttle.http://www.overclockers.co.uk/showproduct.php?prodid=GX-126-OK&groupid=701&catid=56&subcat=1010
Or you could get an HD 4670 less than 60W aswell but I bit more powerful than the GT220.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-209-SP&groupid=701&catid=56&subcat=1274