Thanks for all your replies! I am considering the following Radeon HD 6770:
XFX ATI Radeon HD6770 1 GB DDR5 VGA/DVI/HDMI PCI-Express Video Card HD677XZNLChttp://www.amazon.com/gp/product/B004Z5GRVW/ref=oh_details_o00_s00_i00
It seems a good deal (with the rebate), and quite sufficient for Diablo III. Could anyone confirm that it works with Dell Precision 390 Workstation?