I have Windows 7 Pro 64 installed on my ssd. After installing all the downloads, patches, and fixes it occupies a little bit over 21GB. Others have reported a little less or a little more. A 32GB ssd is recommended.
A SATA 3 6Gb/s ssd will work with a SATA 2 3Gb/s port on a motherboard. SATA 3 is backwards compatible with SATA 2. However you will experience a performance hit. The SATA 3 ssd will be limited to SATA 2 performance.