I bought my wife a WD Sata II 640gb Black HDD off ebay(manufactured in 10-10) for 55 dollars: WD6401AALS .
My question is this: Would a WD SATA III Caviar Blue new drive offer better performance because of the difference between Sata II and III? I know the seek times wouldn't be as good, but loading screens for WoW, LoL or Gw2 might go faster?
I'm not buying her an SSD until I buy one for my system, so that is out.
A single hard drive can't saturate sata2 so it will perfrom about the same on a sata 2 or sata 3 controller. Of course the average read speed on a newer drive will probbaly be higher than the average read speed of an older drive.
The read speed on the older drive is 113MB/s where as a newer blue drive's read speed is 126MB/s. Different drive models vary, so you need to check them out for yourself.