The Seagate Momentus XT are great laptop drives, not so great for desktops. For the same price you can get a Western Digitial Caviar Black with greater capacity and performance. The SSD cache won't be noticeable on a non-system drive
Yes it is possible to put any number of drives in RAID and have non-member disks still function correctly. If you're on an Intel chipset set the SATA controller mode to RAID. This will present all RAID member disks, the RAID volume and all non-RAID disks as AHCI devices. Please bear in mind that SSDs should not be put in RAID at this time due to shoddy TRIM support when in RAID mode.
If you are using an AMD chipset I am not sure what the exact procedure is, but I imagine that it is not that different.