ATi is being very quiet about R300, compared to the leaks we've seen from Matrox and nVidia. Either that means their next-gen part has no where near the specs of G1000 or NV30 or else they got a few tricks up their sleeve. I certainly hope for the latter. I've been a very happy customers of ATi products for over a year now, owning both a Radeon 32DDR and a Radeon 7500. I have no reason not to stick with ATi.
Again, here's my prediction:
G1000 will be the most feature-rich card, but it will be the most expensive of the three.
NV30 will take the performance crown.
R300 will be no slouch, but will be about 5% slower than NV30, but will come with such new features as hardware MPEG decoding. It will be priced lower than G1000 and the high-end NV30 part, but higher than the low-end NV30 part. Simply put, it will offer the most bang for the buck and will probably be my next card.