As long as the throughput is good I guess you're good to go. Usually on an ecommerce machine we were putting SCSI drives at 15K. On some of our other servers we use 160GB 7200RPM SATA's (8mb cache) and we're not seeing any bottlenecks.
I tend to stay with RAIDED SCSI's on eCommerce boxes as there's just so much read/write data that goes on.
It'll be interesting for the future how the Solid State drives play out with speed and buffering.