Motherboards will support however many SSDs of whatever size you connect to them, there are no limitations on number or capacity.
SATA III SSDs will also work in SATA II ports, just at a lower maximum throughput. The lion's share of their performance, however, is in their extremely low latency, which is not impacted by the type of SATA port you use.
In short, knock yourself out, get whatever SSD setup you feel is best for you. Personally, I'd recommend a 120GB SSD with a larger mechanical drive for storage. 60GB is small enough that you're going to find yourself playing the disk management game all the time, and that's not a fun game to play.