The spec sheet says Hitachi HDS721050 CLA 662 SATA disk device. If I don't get any real advantage from SSd what is my best option
They're all "SATA devices". That doesn't mean it is SATA I. If this is your board, it has SATA II. http://support.hp.com/us-en/document/c02966470
Not as fast as SATA III but enough that it would be worth the investment if you want quicker boot and load times. What size would you need?
The spec sheet says Hitachi HDS721050 CLA 662 SATA disk device. If I don't get any real advantage from SSd what is my best option
They're all "SATA devices". That doesn't mean it is SATA I. If this is your board, it has SATA II. http://support.hp.com/us-en/document/c02966470
Not as fast as SATA III but enough that it would be worth the investment if you want quicker boot and load times. What size would you need?