I too have been experiencing crashes with my Radeon HD 4850.
System:
Intel Q6600
Asus P5E3 Deluxe/Wi-Fi
2x1g DDR3 Patriot 533 Mhz
ATI HD 4850 w/ Catalyst 8.10
Corsair 620W
Creative Fatality X-Fi Audio
Windows XP (DirectX 9)
History:
I recently decided to use the ATI CCC Overdrive tool to overclock my card. It OC'd to GPU Clock: 690 Mhz & Memory Clock: 1153Mhz. This caused crashing in all games I play, although crashing occurs at random intervals. When it would crash it would cause a "clicking" noise from my speakers. I "re-set" my clock speeds to the defaults and now I find that the current speeds displayed are rather low: GPU clock: 500, Memory Clock: 750Mhz. I assume this is part of my Mobo "powersaving features" although I'm not too knowledgable on such topics. Sometimes the "Current Values" appear as 500/750, then display 690/1153.
Games that it has crashed on include: Farcry 2, Mass Effect, Call of Duty 4.
Crashes did not occur before I attempted to overclock.
Thanks for any help your able to provide,
MushroomSoup