There is no reason why you can't run SCSI and ATA or SATA in the same computer.  Most people who do that use SCSI for their OS because it's fast, and use the bigger, cheaper ATA for storage.  You might have to play with it for a while to get the computer to boot from the SCSI, but it's not difficult.

The only problem with SCSI (besides cost) is its picky nature.  It has to be terminated properly and all that.  Personally, I prefer to run SATA drives in my RAID sets.  SATA has nearly kept up with the speed of SCSI the last year or two, and it is far cheaper and fool-proof.