Overlay is more tricky to find out. I'll look into it though if I get time - I'm going to be snowed under with work until Christmas now.
Re overlay, perhaps Christian or Ed could email the foreamentioned companies and ask them for support information. Give them a bone - "We'll mention on the site which company was most helpful" and tell them how many users you represent.
Here's a link to the list so far.
The list
Ed: Could zionworx host it without a link from the main site until it is more complete. Just with a link from this disscusion (I'll update the link above if you give me a new URL).
Perhaps we could include desktop PCI cards aswell to help those buying cards and PC's (the cheaper option).
Other considerations will be OS driver support for the features (98se, xp, 200, nt, 95 etc. - some of these you'll be able to rule out simply because they don't support directx). I know for example that some features weren't supported in the drivers on my nVidia G-force4 until a few months after its release. The new driver patch brought them to life.
Nvidia and Ati use a 'unified driver architecture' - essentially a one size fits all driver package, so this should be easier to ascertain with them than with Intel, AMD, S3 and Via.
Another consideration is OEM cards. They use the Ati/Nvidia chipsets but support features in vairying ammounts. I have an OEM nvidia card and something interesting to note is that newer drivers than the one that shipped with the card (although they claim to support it), disable the video out-put part of the card and just throw a cyan screen out. Still working on that problen
Diceman