Upgrading flash -> 32 mb. Cisco 3640

Hi All

I have a C3640 with 8 mb. Simm flash (with an old IOS image 11.2 etc.) I decided to upgrade it with 64 Mb. of RAM, 32 Simm flash, and an IOS image

I tried to install 32 Mb. flash simm first and to load from ROMMON the new IOS image using xmodem commands. But after uploaded the IOS when I tried to boot the router hangs with error messages ".....cannot load .... cannot find the magic number .... and something like that". Maybe the new IOS image is to big (about 25 Mb.) but the total amount of the flash is 32 Mb. Should I replace the the rom chip too ???

Thanks all

Piertonio