Hello,
the 3640 has two banks for 16MB Flash memory each, so the MEM3600-2X16FS (which is exactly that, two 16MB Flash SIMMs) will work.
Check the link below for a layout of the memory slots on the 3640:
Cisco 3640 Router (flash simm memory)
http://www.memoryx.net/ci36rofsime1.html
Regards,
Nethelper