05-02-2012 11:49 AM - edited 05-02-2012 02:15 PM
This user reports that the Sapphire AMD Radeon HD6670 Low Profile 1GB GDDR5 works, but the plastic hard drive bracket needs to be modified for it to fit properly.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102958
http://www.tomshardware.com/forum/347952-33-hd6670-profile-lonovo-thinkcentre-m91p-work
