AMD Says 55-68C for the 4800+:
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I don't know why they give a range - it seems really stupid to me, instead of just giving me a cutoff for safe operating temp.
Regardless, I don't think that's the issue, because it doesn't sound like a heat lockup. CTD is usually a compatibility issue - heat lockup usually locks/crashes your whole system, forcing a reboot, and usually with either some visual corruption or a completely frozen screen. Got anything running in the background? Try the 169.28 beta driver from NV - it was released specifically to fix some issues with Crysis and SLI, maybe it'll help you out too.
You do have Crysis patched to 1.1 yes?
Sound drivers up to date? Motherboard drivers?
Other things you might try:
-Drop your OC say 200Mhz. Could be the memory controller can't handle Crysis at its current OC.
- Loosen your memory timings a tad. Again, the RAM may be getting hit harder than it has before.