all modern motherboards have sATA II onboard, fortunately this type of connection starts to become available also as eSATA (enhanced, external) and it is the logical successor of SCSI - either regarding speed and performance. one step further is SAS (serial attached SCSI) where 15k rpm drives are already available ... most SAS controllers accept both: sATA and SAS disks.
i can only recommend the usage of sATA disks and we already have a pile of 1 TB seagate and samsung drives in use ... the raw throughput is about 70 MB/s (sustained, not peak) ... as written in earlier posts i don't see a real reason for raiding such disks.
just make sure your sATA expansion card is sATA II (3 Gbit/s) and whenever possible use PCEe or PCI-X types - in this case *normal* PCI (32bit) would be the bottleneck ...
christian
ps: another advantageous sideffect: you won't spend CPU for firewire ....
edit: a quality mark is mentioned capabilty of NCQ (native command queuing) and/or TCQ (tagged command queuing) - either for the controller and for the disks
and remember: only a CRAY can run an endless loop in just three seconds.