JustMyTwoPenceWorth - those aren't quite the right definitions.
An Atheist doesn't beleive in a god.
An agnostic doesn't believe there is currently any evidence for the existence of god.
I don't believe in a god (atheist), because there isn't any evidence (agnostic).
So I'm an Atheist Agnostic.
If someone believed in god but doesn't believe there is any evidence of god, they would be a Theist Agnostic.
If you beleive in god AND that there is evidence that god exists you are a Gnostic Theist.
If you don't believe in God but do believe that the there is evidence that god exists, then you would be a Gnostic Atheist - but that would be an odd position.
In summary Atheism/Thesism - what you personally beleive.
Agnosticsm/Gnosticm - what you beleive there is evidence for/you have kowledge of.
Many faiths consider the 'revealed' or 'insprired' word of god to be evidence of gods existence - so they are Gnostic Theists.