Hi Steve, welcome to Tom's Hardware,
I assume you have your Primary system 3 Gb/s SATA drive plugged into the Intel P55 PCH SATA II Blue connector on the MB.
You can use the new SATA III 6Gb/s SATA drive connected to the Marvel controller SATA_6 or SATA_7 Grey connector which run at 6Gb/s or one of the other SATA II Blue connectors where it will run at the slower #Gb/s speed.
You will hear some folks disliking the Marvel 6Gb/s controllers on the early 1156 boards, because the PCH did not have the faster SATA connectors, but most other board manufacturers have used them, still do, that there can't be anything really wrong with them.
Your new HDD is the faster 6Gb/s SATA III type, and it will run at the slower 3Gb/s speed connected to the PCH 3Gb/s plug, but if you are using it for secondary storage, you won't see a real speed difference. It is backward compatable.
I am building a new machine, using the asus P8Z68V LE mobo, and am planing on using my existing drives, one of which is a parallel drive (using the old wide ribbon cable). The others are sata drives. This mobo onlyhs connectors for sata 3.0 and 6.0 cables. Question is, can I use the parallel drive on this mobo, using an adapter that changes the parallel connector to a sata type. I assume that it will not run at either the 3 or 6 GB rate (depending on which board connector I use), but will function as well as it did on my old mobo.