Sorry if its in the wrong forum, but theres no dreamcast hi res texture forum!
Did you know it is possible to replace the textures in a lot of dc games through the use of umod - http://code.google.com/p/texmod/
it allows you to extract and replace textures, even with the game running, all you need to get it running is add nulldc to umod, set global hook, then load nulldc to see if it worked, and setup umod.
the picture above hopefully should give a rough idea what can be done.
It is also possible to replace sprites, even to go as far as turning them into non palleted textures, which means you can increase the number of colours, and detail, although its a case of trial and error. (see attatched image)
finally i've included a sample of the current wip, for ferrari f355 pal dreamcast, for anyone who would like to see it in action for themselves, just unzip the 7zip and load the contained zip file into umod and run the game in nulldc. (which currently is for suzuka circuit only)