Does my laptop have SATA 3 or SATA 2

Yes, it supports SATA III for SSDs

I know you can put SATA III drives into a SATA II but if the computer is made for II then I won't get all the performance out of the III drive that I put in.

Yes, but the 521x will run at SATA III speeds with an SSD. For a SATA III hard drive, it will run at SATA II speeds. Remember, however, that you rarely get max speeds in real life. My E660 has both a 2TB and 1TB SATA III drive. Drive-drive transfers run at 1 GBit/sec, 1/6 the maximum. However, this is great, as transfers with my SATA systems run at only250-300 Mbit/sec.

OOoooooooh so it will run at the theoretical 6 gb/s as long as it is an SSD.  

Yes it will.