The Intel X25-M is the 1st consumer-grade SSD released. It is a 1st generation SATA 2 (3Gb/s) SSD that went on sale in 2008.
The Intel 320 series SSD is a SATA 3 (6Gb/s) released in 2011.
You should consider a current generation SSD such as the Intel 530, Samsung 840 EVO or OCZ Vector 150.
Definitely do not get the X-25M.