Photoshop cs5.1 (12.1) was a paid upgrade from photoshop cs5.
(not really an upgrade because cs5 and cs5.1 have the same features, more or less)
(photoshop cs5.1 was part of the creative suite cs5.5, in which some programs had new features, but not photoshop)
I don't think you would gain anything anyway over photoshop cs5.
The latest update for photoshop cs5 is 12.0.4
Have you used Help>Updates from within photoshop cs5 to get the latest updates for photoshop cs5?