I'm thinking of building a new rig, that will also run some (wine-d) games. Nothing heavy and super modern and graphic intense.
My options are, either keep my old board and use an AGP r500 card, (X1950 Pro i belive) using the OSS r300 driver only. Or, get something new, with PCI-e and use a new budget-mainline graphics card, which be based on the r600+ chipsets.
The upside on a r300 based part is that the driver is pretty much done. The downside is of course, you can't use the catalist drivers.
I know r600 based parts are coming a long nicly now driver wise, and when it's not; the catalist driver still works. I still much prefer an all oss solution.
In terms of functionality, there is not that much missing from r600g, and the missing stuff is close to being done -- HyperZ, MLAA, 2d tiling, texture tiling, etc.
The powersaving is the same on all radeon cards, so a less power hungry r700 or r800 is better than a monster r500. And even with all the extra optimisation in r300g, the pure HW power of the later cards will pull them ahead.
I can't think of anything that r300g can run, that r600g can't right now.