i used to run impulse tracker on a 286 acer for a while with an ess 688 soundcard , running mono, 22.050mhz, with a 20 meg hd. once i would load up all the samples individually in a song, it wouldnt skip or anything. but i had to use 386 memory in there and tape it down and stick guitar picks in and shit in order to get enough ems/xms memory. i even used this mess of a rig for a few shows with no problems surprisingly..think i upgraded one time to a pentium 1, probably 100mhz, then to a pentium 2 400mhz, and im still using it now.
if you want to get into an impulse tracker setup, a pentium 2 is great, cheap and plentiful at thrift stores (just look around for something that looks good and clean, if youre not good with older pc's take someone with you who is) with that mmx shits and get a sblive in there, sb live will cost you no more than $5 now. do not even attempt to fuck around with configuring IT with any other soundcard really, its not even worth the trouble. 400-600mhz is basically what you need to get going. also, i donno if schism tracker's mouse functionality is different at all from impulse tracker's, i never noticed any differences.. and besides, the only thing you really should be doing with a mouse in IT is drawing envelopes, you want that intuitive, sorta ergonomic hotkey-dependant setup there.
the biggest problem schism tracker has going against it is that a lot of key combinations can get 'stuck' on ya. its too buggy and they put too much effort into it for being as buggy as it is. samples sometimes will not trigger at all if they're below a certain samplerate. the thing about schism thats a really strong thing in its favor, its useful to learn the ropes of getting into a tracker like that without having to go and make a true dedi IT rig. its gonna sound different and better and after you've used schism for a minute, you're gonna say 'wow, IT is a lot...stronger...or something?' and you want to do that if you try schism and enjoy it, find a good p2 that you can throw a big hd in and a sb live in and have an enjoyable time in bringing some new life into an old box.
perhaps you might not even run into some of the problems that some others have with that program, depending on the good or bad habits you pick up along the way in tracking. i sort of think laptops are out because while you can use external keyboards (and really, tracking on a tiny keyboard gets tiring fast), you arent gonna get the ideal sound quality that you'll get with an sb live, which is pretty much the end of the line driverwise with that program, and lets be honest, thats the big draw in using the program in the first place. it sounds way better than with a regular soundblaster.