SATA2 & SATA3

[HairyScot]

I have a PC with a GA-B75M-D2V MB which has five SATA2 ports and SATA3 port.
It came from the shop with the SATA3 port connected to an external ESATA port. I'm not sure why.
In any case, I intend installing an SSD and my feeling is that it would be best installed on the SATA3 port.
Is that a valid assumption?

I don't envisage ever using the ESATA port, but if I did would it function with SATA2?

[shadowsports]

Your assumptions are correct.  Connect your new SSD to SATA3 port if its a newer disk that supports this standard.  Anything else, CD/DVD as well as eSATA can connect to SATA2 (ports 1-5)