it's interesting how for some reason, there aren't any NORMAL mass storage type drives available in SCSI form.. that would be nice having IBM making a SCSI mass storage drive (like a 75GXP, only SCSI!). the important stuff (OS, other stuff needing fast access) would go on the main SCSI drive, then the MP3's, some games, etc would go on the mass storage drive!
anywho, if I were in your position, I would not worry too much about space, becuase a CD Burner for me, solves almost all my space problems (I'm running a 4.3 gig IDE drive here). I just know how to control the amount of crap (shareware proggies etc) that comes on here, and am able to keep Windows itself, down to a manageable size for longer then many hardcore users (I have a friend who knows jack about computers, he has to reformat like every 3 to 6 months! LOL, but he uses it all the time! you'd think he'd learn at least a little bit!). the only thing that REALLY takes up space in the end, are games.
now, also, I have learned over the past little while, the SCSI is alot better then IDE in terms of CPU usage. that counts for a lot with me. So if I were to get a SCSI harddrive, I'd go ALL SCSI, and disable my onboard IDE stuff (the IDE controllers take up IRQ's!). that limits me in terms of the amount of overall hard drive space, but, if I can live ok on a 4.3 gig hard drive, then I think 9 gig's is heaven!