Oh so very possible, your onboard card should still be in device manager (right click on "My computer" and click properties, then hardware, and device manager. Find the default card, and find the setting that says to hide this device in all profiles. If you turn it so it can be seen, right click on My comptuer, and click settings, then click on the second monitor, and check a box that says something like "extend windows to this desktop"!
And it should work.... i have a 512 meg x1800xt, and it has dual dvi slots, and an svid so in theory yours should work, (i stated what i have to say i havent done it, but its theoretical)
Mac G5 workstation with 4 gigs of ram, 4x250 gig drives in raid 5. High availability cluster server running centos for a server. 2xCanon GL2 cameras.