Yes, SATA 3 (6Gb/s) SSDs will work on SATA 2 (3Gb/s) ports.
SATA 2 speeds are from 151MB/s to 300MB/s. With a SATA 3 SSD on a SATA 2 port you should expect Read/Write speeds up to around 290-295MB/s.
You should save your money until you can afford a 120GB SSD. A 60GB SSD will be 55.8GB after you format it. A Windows 7 installation with all of the updates take about 20GB. So you will be starting out with 35GB of available space before you even add any programs or games.