Riva TNT and ATI Rage cards and older do not support DXTC.
Add to that list Matrox Millennium G550 and Matrox Millennium G450, the problem with these cards is from my experience they never die...
If you do want to target ancient computers then I personally would be looking at targetting a 400mHz Pentium2 with a 16MB 3D card without DXTC. I've tested games like Platypus and early versions of Cletus and both worked fine on this configuration.
It is a minimal market that only a minimal number of Blitz3D programmers will be interested in but I don't think that makes it any less legitimate.