I'd personally recommend nvidia cards for their Twinview but you're extremely limited in what you can buy for AGP. The 6800 GT is probably the best card you can get fulfilling your requirements. Helpfully it's also supported by the current driver. I can't say how long it'll be until Nvidia stops supporting it but it's probably quite soon.
Just to make things even more annoying, AGP Nvidia cards are at that point in their life cycle where they're becoming more expensive than they should be, just because they're so rare. Damn you, market forces!
It seems odd to say it but binning the old computers is probably the best long-term plan in terms of price, performance and support rather than spending £50 per card for technology that is 8 generations old and might not be supported in the future.
If you need to evaluate it, you need to evaluate it. Buy one, chuck it in one of your machines and give it a spin. If it doesn't work, you've only lost £50 and the 20 minutes or so it takes to throw it in and test.
Alternatively there are devices like the Matrox DualHead2Go and TripleHead2Go that combine two screens into something the system sees as one monitor. They're not cheap and the computer won't know how to make things fullscreen as it would in a "proper" twinview setup, but it's another option if you're desperate.