There isn't technically any such thing as SATA II. It was the name of the SATA International Organisation before they changed their name. One of the features that the SATA II organisation came up with was an increase in speed for SATA drives from 150MB/s (or 1.5Gb/s) to 300MB/s (just like with UDMA133 over UDMA100 with IDE drives for example).
End result, people see "3Gb/s" and "SATA II" and think "Oooh, new standard", despite the SATA II (now SATA IO) saying, "
No, that's just our name".
End result. SATA is SATA. Newer drives support newer features such as the 3Gb/s transfer rate, but everything is pretty much backwards compatible because SATA II doesn't technically exist as a drive standard. Hence also why you'll struggle to find any drives listed as SATA II on sites such as Seagate's website or Western Digital's website.
The only occasional problems are a few earlier motherboards that don't support the "Auto speed negotiation" feature and hence can't tell the hard drive to run at 1.5Gb/s. On these such boards, you may need to jumper the drive to force it to run at the slower speed. On all other boards though, everything will work fine.