Ps2 will *maybe* have a 2 month window where its graphics exceed the PC's. Thats a BIG maybe. Graphics on the PC just get better much faster. Remember when the original playstation came out its graphics were much better than doom, or duke or whatever else was out there on the PC. But when GLQuake and such came out, the PS was blown out of the water. Nowadays PC graphics development is going at an even faster rate. I remember buying me Geforce2 thinking I'd have the fastest vid card for at least a year...within 3 months the Geforce2 Ultra came out. Any lead the PS2 ever has on the PC will not be for more than a few months. The PS2 will be around for 3 or 4 years before new hardware from Sony comes out, while PC games look better every 6 months.
As for the dreamcast, its much easier to develop for than the Ps2. In order to use those vector units you're talking about, you have to do all the programming in assembly code. Don't just assume that "oh yeah the programmers will just figure it out and it will be all good." Programmers are human beings like you and me. The problem with working in assembly code is not so much difficult as it is time consuming. If it takes them 5 days to make a cube move in three dimensions versus spending 5 minutes doing it on DC, which do you think they'll do? The one that allows them to develop faster and more efficiently. I'm not saying the PS2 is gonna fail like the Saturn did (incidently the Saturn used assembly code too), but its definitely not gonna blow the Dreamcast out of the water. Sure, some PS2 games will look a lot better, but those will be few and far between because of the high development costs.
As for me, I'm happy with my PC and my dreamcast. When the X box comes out, I'll definitely get it since it'll have the power of the PC with the developers of the console. Just my .02