Buy a two Momentus XT 500Gb drives and run them in RAID0
The other option is to buy an SSD such as the OCZ Synapse Cache and then buy the Caviar Black. The Synapse SSD works like Intel Smart Response on the z68 platform.
The Momentus will be faster because it is a HDD with a SSD cache built in. The problem is that neither of these drives will saturate the SATA III bus, but may saturate a SATA II.
Any SSD set up to cache a HDD will be slower than the SSD run on its own. A best performance scenario is to set up the SSD as the boot drive and place all apps that need quick loads on it and use a HDD for storage and apps that do not need quick loads.