think, the only difference between the 3640 and 3640A is the PCI bridge version.
you may use the show version command, the platform would be displayed in this command output.
3640A
-----
cisco 3640-A (R4700) processor (revision 0x00) with 91136K/7168K bytes of
memory
.
3640
----
cisco 3640 (R4700) processor (revision 0x00) with 93184K/5120K bytes of
memory.