Did dreamcast have only 1/3-1/4 the power of the ps2?
Do these comparisons really matter?
The 128 bit generation was pretty much out there when it came to hardware parity of features, it was pretty much a matter of pros and cons of each architecture really, although GC and Xbox were in a class of their own. PS2 was an alien system with the vector units and a really atypical GPU that didn't support on-silicon acceleration of certain effects and techniques like DC did.
DC had a GPU with some performance advantages over PS2's custom GPU, namely the fact it could render in tile-based form (meaning only the visible pixels in the frame were textured saving calculations and texture-bandwidth) and it had texture compression which PS2 lacked so textures for games like Shenmue are actually really good for the time, unlike most texture work you'll see on the PS2 which by comparison was plagued by low res and low color depth on it's textures. I mean most of it's games stuck to 16 color textures, at most. (GTA's being really good if you want to be able to count the exact amount of colors per texture because they were so clearly unoptimized)
DC was a texturing beast, meaning texturing (also because of the tile-rendering) for a whole scene would take little hit compared to PS2, that was it's advantage.
The disadvantage? Well, it's specs claimed 3 million polygons real time; because of the low hit texturing had they were able to pull as much as 4.5/5 million polygons per second in Test Drive Le Mans; PS2 on it's turn did manage to pull the 10 million polygon mark on some games (Jak and Daxter, Virtua Fighter 4) which was the max polygon throughput in real-world really. If we're talking at straight Raw figures then DC was roughly 1/4 of the PS2 in total Gigaflops, but it was really efficient with it, so it closed the difference a little bit to 1/2, as polygon throughput figures put it, and it actually humiliated PS2 on texturing. Because of that, PS2 spent most of it's life pushing far more polygons than the DC could muster and ridiculously bad textures to go with it.