ReadyNAS Ultra 4 Plus

Personally I wouldn't like to take the risk on 7,200RPM drives in a NAS. The extra rotational speed your unlikely to max out on the Ultra's GigE interface, and the extra heat from the higher rotational speed may reduce the life of the drive. In answer to your question about 6Gb/s, yes the drives will work, but AFAIK the ReadyNAS SATA interface is SATA-II standard, not SATA-III.

After all the controversy over Seagate's HDD's and firmware bugs causing critical drive deaths on a number of their drives over the past year, I wouldn't touch a Seagate drive with a 100ft flag pole at the moment, but that's just my personal opinion. I have 4x Samsung SpinPoint F4 (2TB) drives in my NAS that (touch wood) haven't given me any issues. I would wait for a 3TB drive from them or WD if it were me making the choice, but at the end of the day, it's down to what you feel comfortable using. Also, i'd reccomend checking out the HCL before planting any cash.