This should be in the gaming section...
Anyway. beedubaya, this has nothing
to do with your hardware, before you start cursing your X800 Pro
(or your RAM or anything else...).
I used to get the same error message. Many others on these very forums have got the same 'Could not be read...' error message.
When was the last Steam/HL2 update you had? Up you're up to date, then I suggest a re-install, as you're probably looking at a dodgy Steam update...
EDIT: I've always found HL2 just doesn't like CPU/RAM overclocking whatsoever. Undo any adjustments you might have made in this department.