I get constant crashes using 4870 radeon and windows 7 this only started when I upgraded from an nvidea 8800 gt. Things that cause it to crash for me are  poser figures and especially when i move the animation timeline bar too fast.

Once it has rendered more than 4 frames I can pretty much guarrantee the scene will be stable for the whole render.

My best advise save every few minutes and if the scene crashes... reload and change the materials slightly this usually fixes for me (even if the material you change too is more resource heavy).