I know some AMD boards allow onboard along with an add in card, my brothers last 2 AMD systems allowed this, he ran 2 monitors off of an 8800gt ad his tv off of the onboard hdmi, and his other system had a gtx260 for the monitors and the onboard hdmi for the tv.
I asked him if there was a bios setting to enable both and he didn't remember.
I guess you can go into the bios and search fir the onboard setting and see what options there are.
With Windows 7, you can run different brands of cards at the same time, so you could just pop that ati card in it "should" work.
If none of those options work for you, you could get one of those USB adapters that run a screen. They are about $35 to $70 depending on make.
I used 2 of the Startech branded ones for a client to hook up 2 dvi monitors to his laptop so he could have 3 screens. http://www.amazon.com/StarTech-com-E.../dp/B002ZT08U6