Dual displays with older hardware

Looking for a *cheap* solution here. I have a reasonably modern but low end PC whose motherboard (VIA P4M800 Pro chipset) supports AGP and PCI slots. I'd like to use a dual display setup on this motherboard, ideally without upgrading to a new primary graphics card.

I'm using the sucky built-in S3 Unichrome graphics for the primary display, and as this box is for office-type work, it's generally fine. To add the secondary display, I tried using an old PCI ATI Rage 3D card, but the system became unstable, locking up regularly, especially during even slightly graphics intensive operations (e.g., running the "3D Pipes" screensaver).

I've also tried using an even older S3 Trio card (Diamond Stealth 64...!) with very similar results.

OS is XP, and I have duplicated the problem on a clean install. I'm using the drivers supplied with the motherboard. I tried upgrading the display driver to the latest version from Via's web site, but when I tried booting with that setup the display didn't come up at all, I only got the old PCI card.

Am I trying something that's impossible here, or should this work? Any suggestions of how to get it working, or even what might be wrong with it...? I know its a lot of effort to go to to avoid spending thirty quid on a cheap dual output graphics card, but my budget is somewhat reduced at the moment so any help would be appreciated.

The BIOS seems to believe that the IGP is an AGP card: as I understand such things, if I put an AGP card in the slot, the integrated graphics would automatically be disabled, as only one card can connect via AGP. Hence using PCI cards, not AGP ones... except the damned thing is unstable.