Vapor-X keeps the card a bit cooler than the regular one. Vapor-X also comes overclocked as the guys above me mentioned but it's also more expensive than the regular one.
Actually, it doesn't.
There were tests done on OCN.
http://www.overclock.net/ati/651602-contagion-review-sapphire-vapor-x-radeon.html
It does have better memory than the reference HD5770.