It appears your board (2AC8) is terrible about upgrades.
Lots of good reading here:
https://www.google.com/?gws_rd=ssl#q=2ac8+video+upgrade
I think what needs to happen is a bios update for your board to make other cards compatible.
This thread has a resolution:
http://h30434.www3.hp.com/t5/Desktop...d/td-p/3108285
The link posted to the documentation is broken on their page, here is a fixed link:
http://support.hp.com/us-en/document/c03092621