Your best bet apart from using chris' config, and -dxlevel 81 in the launch options, is trying to optimize your PC outside of the game. Like some other already mentioned, disable windows' services that you don't need (there are quite a few of those), disable the visuals (aero and all that shit), defragment your HDD, and after that also defragment the game cache files (not through the steam defragger which is known to be pretty weak to say the least, but using GCFScape, here you have the how-to steps http://forums.steampowered.com/forums/showthread.php?p=18225610 although you can try googling for some better source if you have dificulty doing it).
Also, install povohat's surfaceproperties.txt which will help you on crowdy fights like middle (it disables the dirt flicks from walls and floors when shooting at them).
Playing without the viewmodel also gives a tiny fps boost, and net_graph 4 (iirc number 3 too) eats around 20 fps, so if you play with that on, turn it off.

Samurai Champloo is great. If you end up liking it all once you finish it, try cowboy bebop. It's a bit older, same director, arguably better than Samurai Champloo, music still awesome, a classic.