Mine is not so old : debian jessie, with kms and agp mode enabled. 3D works reliably and is very stable on this powerbook G4 1,67 and radeon 9700. It's not bug free, mostly endianess problems with colors. Not all programm using 3D work, but kodi does very well.
Overall, the whole interface (mate) is much more fluid than without 3D acceleration. Without it i slow.
I didn't install latest unstable yet because this system is very stable and enjoyable. I did it on a powermac G5 and could'nt enable agp mode because of a bug in the driver (already reported here). Without AGP mode, mate is painfully slow.