* If you plan to use software RAID, the quad 9.1G drives (1st-gen X15's, 2nd gen start @18 and 3rd gen @36) will outperform a single 36G drive. But at a price:
- heavy CPU load
- possible only with NT (and 2000, try to avoid XP...) or Unix / Linux
- lots (!!!) of noise. It will sound like a jet aircraft turbine (1st gen drives...)
- lots of heat (same reason)
- heavy PSU drain
- depending on your RAID level, reduced capacity (for example ~24GB with striping mirroring and parity)
If you can cope with that, and maybe pick up a RAID controller somewhere you'll live happily in SCSI-RAID land.
* If you plan to use them as 'standalone' drives, to get a total of 36GB you're better off getting that new X15 (it's a X15.3, right ?), by itself it's faster than any other drive, including previous generations of X15's.
- it's silent (3rd gen drive...)
- its blazingly fast
- it's slim (only 1 drive to fit..)
- it wont kill your PSU
- its rather cold (for a 15K HDD...)
Besides you've already got the adaptec board.
To me the choice is rather clear: get that X15.3
