Dell Precision 690 - 2nd Video Card

Hi all -

My friend's church recently inherited an in-good-shape Dell Precision 690 workstation which they intend to use for AV processing during their worship services. I agreed to help set the machine up, and in the course of discussing their needs, I realized that the computer will need a second video card to facilitate what they'd like to do. My question is, basically, how best to do this without blowing huge amounts of money.

I know from scanning previous threads that the "official" way to run dual graphics on the 690 is to install Dell's graphics riser card, then add a second PCI-e card to the additional x16 slot. I'd love to do this if possible, but I'm not sure the cash is available to splash out the extra $100 for the riser board. What I'm wondering instead is if I can use any of the other existing x4 PCI-e slots to just drop in a second video card without adding the riser; I know this won't yield optimal performance, but is it even physically possible? Or is there any other alternative I've completely overlooked?

Thanks in advance to anyone who can provide me some input and/or direction on this--I really appreciate it!