What you're looking for is called a JBOD (Just a Bunch of Disks). It's a simple type of RAID array that has no redundancy. Never used one myself, but I imagine it would have all the drawbacks of RAID 0 with none of the benefits. Namely, lose a disk in the array and your whole array is probably hosed.
Personally, I'd just deal with the disks as they are. I segregate my data by folder anyway, so you may find that it's simple enough just to dedicate the 160 GB drive to one type of data and the 750 GB drive to another. The SSD should obviously be your system drive, 120GB is plenty for Windows and a couple big games.
In fact, I have the exact same SSD and use it for Windows and my two most frequently played games, along with all my small Steam games. You can move Steam games to and from the SSD easily using
Steam Mover, a simple but very handy piece of software that creates symbolic links automatically so Steam can always find the games regardless of which drive they're on. Plus it handles moving the games between your drives easily.
My personal setup is a 120GB SSD, a 60GB SSD (bought it for my wife and her POS motherboard won't recognize it), a 400GB HDD, 1TB HDD and 1.5TB SSD. Media goes on the 1.5 TB drive. I rip all my games to ISO (installs faster after reinstalling Windows) and those go on the 1TB HDD, along with a bunch of miscellaneous crap. The 400GB drive is where my installed games live when I shift them off my SSDs.
I use a lot of storage...