So, I didn't play Psychonauts until last month. It runs silky smooth on my 5-year-old machine (high-powered machine for the time). So your problem isn't the game, it's some interaction between the game and drivers, or something. Maybe take it for a spin on a crappy laptop or something. ;)
And this game has no nostalgia for me, but it is amazing. The gameplay, which is mostly 3d platforming, isn't the stand-out (though the gameplay is better than I expected). The amazing thing is the writing, both in terms of dialogue/voice-acting, and in terms of the plot. So awesome. It's surprising how few really good writers there are in gaming, and how most gamers apparently don't even notice, and so the standard is so low. And then you play a game written by Tim Schafer and/or the OMM guys, and boom.
This game gets so much nostalgia cred, someone should really clean it up.