The problem is that when a developer tends to develop a game multiplatform, they tend to go for the PS2 as the lead version, and that's why a lot of GC and Xbox versions tend to look close to the PS2 version. I'm personally getting a bit sick of that. It would be nice if they started developing Xbox down to PS2, that way you can dumb down the textures and effects with each version you do.
There are some occaisions where the Xbox version (or GC) will look better than the PS2 version, and that's pretty much when the releases are staggered. Look at Pandora Tomorrow (or the first Splinter Cell). It was developed and then published on the Xbox first, and as a result, it looks better, because the developers designed the game around the Xbox hardware. Then they can go back to doing the GC and PS2 versions, and work around their hardware, so at least the game looks pretty representative of it's technology. Then as we saw in Burnout 2, the PS2 version was released first, then the GC and Xbox versions came out 5 or so months later with improved controls (I heard that the PS2 version had a "dead zone" in it's steering/turning), sharper textures, better particle effects, better lighting, and a more solid framerate.
So, the real problem is that when you have multiplatform games, most of the time the PS2 version is the lead version (as it will probably be released first and make the most money). Only when a developer takes more time to optimize the port is the time when the game won't look like a PS2 game on the system. At least there are the console exclusives made available specifically for one console, so at least there's something to show what a system can do, and that the game will look better than most multiplatform titles.