You should most defenitly use the SSD as the OS drive since that would be the best use of it. The fact that it's 180gb will allow you to put the OS and some other things on it and you want to be saelective as to what those things are. A couple of games and your most used program , with the other stull on the secondary drive. Keep in mind that you don't want to fill the SSD past 80% capacity because if it's too full it will affect performance.
Before you load the operating system on the SSD make sure you change the sata 3 mode in the bios to AHCI , some motherboards are now coming with that enabled by default but just to make sure. Also connect the SSD to the Intel controlled sata 3 port.