The serial ATA idea is good. That is what I have done, with two 200GB drives. Although I don't use the mirror raid, I use stripe for fast access because I have large files to work with: 500mb to 800mb scans of X-rays in some cases.
Really, 400gb striped should be okay for most people. Then I have a 140gb external firewire backup drive, and two internal IDE 120gb drives for the system. Lastly there is a 1gb SCSI external drive that was setup as FAT95 so it could be plugged into the A4000T or the PC (but I need to format it agian).
I think it is a good idea not to mix Windoze, Backup, and graphics-currently-being-worked-on files. Hell, I don't even let them sit on the same controller.
