The current "sweet spot" is a 128 GB SATA 3 6Gb/s solid state drive.
Typically a user will install the operating system, software applications, utilities, and some favorite games on the ssd. Data and additional games are stored on a hard disk drive.
I normally recommend Samsung ssd's due to their proven track record. Crucial, Intel, and Plextor are also worth considering.
I maintain the ssd database listed in the sticky at the very top of this forum section. Here is the link:
http://www.johnnylucky.org/data-storage/ssd-database.html
Scroll down to the brands and models you are interested in and follow the links to the technical reviews.
Best bang for the buck is probably the Samsung 840.