SATA II is actually backwards compatible. Therefore, it doesn't really matter what you get. Also, IMO, you won't see an improvement in performance if you go SATA II. Really, what it comes down to is what kind of price you pay. If you can somehow get a SATA II HD for cheaper, then get it, but I wouldn't spend any extra on one.
Also, I use and recommend Seagate and Western Digital HDs due to their warranties and relability.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64