Get a SATA 3 HDD even though you will be connecting it to a SATA 2 port.
Even though a SATA 3 HDD cannot spin fast enough to achieve SATA 3 speeds, it is still faster than a SATA 2 HDD.
The 1TB and 2TB HDDs from Seagate have Read/Write speeds of 210MB/s. http://www.seagate.com/internal-hard-drives/desktop-hard-drives/desktop-hdd/