Hi there,
The Western Digital WD20EARS 2TB Hard Drive SATAII 64MB Cache - OEM Caviar Green Hard Drive is a "middle of the road" drive in relation to speed. It slows down and speeds up to save energy, ie "green".
If you are looking for "fast" in a HDD, the Western Digital Black WD2002FAEX 2 TB SATA III is faster. It is a SATA III drive, however your MB only supports SATA II (3Gb/s), but would be ready when you do get a MB supporting SATA III drives. Overall it is cheaper than buying a drive now and a second one down the road. The "Black" is faster than the "Green", irrespective of the SATA II or SATA III engineering.
If you are looking for "really fast", consider a SSD, which performs 5-10 times faster than the HDD models.