Help needed on Crysis please....

hi

My PC config is Sapphire HD 5850, Phenom IIx4 955 3.2 Ghz , and 4GB 1333Mhz dual channel RAM.
I was playing crysis very high DX 10 at Windows 7 64 bit (1680 x 1050, 2x AA) without problems and smooth frame rates.
but at the last stage "Reckoning" where we have to fight the big alien tripod , my game is getting crashed seconds after i load it and i have to quit
the game   . immediately after i quit ,when i invoke the task manager i found the memory utilization is high at about 3.9 GB.

1) i lowered settings from very high to high and turned of AA , but still the same result.
2) Is this some kind of a bug with win 7 64 bit or crysis ? i didn't install any patch. its a retail version.
3) because my config should be able to handle crysis in above settings quite well, and i had a very smooth gaming in the earlier
much demanding stages of the game , i dont understand why it is crashing at this stage.
4) other games are running fine (i.e) 60 + fps in dirt 2 , 150 + fps in bioshock etc

please help