RE: 3d accelleration support

> If I were to replace this card with another 3d card,
> what would be a safe chipset to select if I want to stick with
> open source drivers?

Of course, publicly availabel documentation has significantly
decreased over the last decade. To my knowledge there are only
few documents availabel at all as of today.

That far as it concerns XFree86 there are severeal OpenSource
driver projects running for current hardware, namely for the 
ATI Radeons, for the S3 Savage, for the SiS, for Chips&Techn.,
for some Intel integrated ones, and possibly a few more vendors.

For 3D you should have a look at http://dri.sf.net in order
to get a current view on what is availabel for your purposes.

-Alex.