First my specs,
I have an I5-3570K, GTX 660, 8GB RAM, 250 GR VelociRaptor with my OS, and a 1 TB HDD in a micro ATX case.
I have been contiplating buying an SSD now that prices are dropping on them. I would put my OS and some of my Steam games on it such as ARMA 3, maybe Rome II Total War...