Amperage discrepancy on computer PSU
Since you're re-purposing a computer PSU, beware that some of them won't regulate properly, or may misbehave in other ways, without a load on +5V too.


  My question is; if I was to power something using a +/-12V rail voltage what would the maximum ratings of the supply equate to?


That depends on whether your load is equal on the +12V rail as well as the -12V rail.


  If the -12v rail can only sink 0.8A does the entire +/- rail supply then limit to 0.8A and become a 9.6W dual supply (or exceed this and damage the unit)?


If your load is equal on the +12V rail and the -12V rail, then yes, the -12V rail limits what you can draw in total.