I use to be a hard core Western Digital fan but when 2 of my 300GB VelociRaptor drives packed up within 6 months of each other I moved to Seagate, have 2x 1.5TB drives in my DNS-323 and one of these did pack up after 1 month due to heat issues, but the replacement has been in there for 6 months get trashed and am having no issues.
I have 2x 1TB Seagate ES drives in my destop running raid 1 and they seem to be working fine.
So go for the Seagate ES editions, any drive models ending is NS are targeted towards Enterprises which will last a lot longer but they are twice the price.
I think the 5400RPM drives only run 1.5GB SATA I not SATA II, but correct me if I'm wrong.