when i upgraded my system 6 months ago, I switched from SCSI to UIDE. I bought 2, 40gb ultra drives and RAIDed them so that one is reading while the other is writing (RAID 0).
sometimes, I have to perform the compact audio data feature in sonar or the app will attempt to read and write from both disks at the same time (i am assuming this). once i do the compact audio, i'm back to blazing away.
the combination of the 1.2ghz cpu, 640mb ram, and the RAID0 disk array has given me a really sound and stable system that i haven't been able to push beyond the limits of with the work that i do musically.
having said that, my wife just bought me a dell inspiron for christmas with 1.2ghz, 48gb hd, and 512mb ram. i'm thinking about getting VXPocket or something so I can use it as a mobile recording DAW.