My Gigabyte motherboard has an mSATA connection for an SSD drive that can be used in conjunction with a standard hard drive to improve performance. The mSATA connection is at 3.0 GB/s. The mobo also has SATA III connections which run at 6.O GB/s. Which is faster -- 6.0 GB/s 7200 RPM hard drive connected to SATA III or same hard drive used in conjunction with 3.0 GB/s SSD?
I would hate to spend money on the SSD if the decreased data rate of the mSATA connection would actually make the whole thing slower.
The fastest overall would be get a 120GB or larger SSD and plug it into a SATA 6.0Gb/s port, install windows and all programs on it, then get another spinny disk drive and use it as a storage drive.
SSD Caching (mSATA + HDD) will be faster than just a HDD, but it doesn't really take effect until you "train" the SSD on what programs you use the most so it can cache them on the SSD instead of leaving them on the HDD.
Even if you plug a 7200RPM drive into a SATA 6.0 port it will only perform as quickly as the drive limits. You would see absolutely no difference in performance if you plugged a HDD into a SATA II or SATA III port.
So, just to reiterate...
1 SSD 120GB or larger in a SATA III port with OS and Common programs installed.
1 HDD with files stored (Pictures, videos, stuff you don't need all the time)