Hi Simon,
The Mac OS X 10.8.3 update resolves the 3D graphics crashes in this thread (even though it is not mentioned in its release notes). The feedback we're hearing for this Mac OS update is overwhelmingly positive.
The Fusion 5.0.3 update is an incremental update with very few targeted fixes (see its release notes).
Noting that installation of any update involves an element of risk (and should only be performed with a backup of any critically important data, and not right before any important deadlines!), my recommendation is to install both of these updates as soon as is reasonably convenient.
Cheers,
--
Darius