What specs for an SSD should I look for?

I own a August 2009 MacBook Pro 15” and I want to upgrade the storage to an SSD. What specs do I need to look for? 2.5”? Intel Chip? Serial ATA? I’m just not sure what exactly is the best.And are there serveral iterations of the transfer speeds like 3Gb/s 6Gb/s. I’m not sure what my MBP will handle. Also i’m looking for storage of 150Gb and up any info or input would be great