i promise not to remind you that i brought this up prior to your purchase =)
I am surprised however that you can get the DVI and HDMI both working and not VGA + HDMI or VGA + DVI. normally its one or the other digital plus the analog that work. but this depends on what your trying to do, extended, stretched, mirrored displays.
you mentioned you were using vga cables and dvi to vga adapter, do you not have a DVI cable? I would borrow or buy a dvi cable and try it first with 1vga cable, 1dvi cable, 1 hdmi cable. granted i dont think it will work but in the end your better off with the dvi cable.
no you aren't screwed, luckily the board has a second pci express slot.
you do not need an exact matching card to make it work either but i would get an ati 5*** series card, the 5450 is the cheapest i think. pay attention if you need a low profile card.. if its the slim case you do.
the 5450 and 5670 will run on the same driver and catalyst control center.