PCMCIA Flash Card
The Cisco IOS software image can also be stored on a PCMCIA Flash card on either of the 2 available PCMCIA slots provided on the Cisco 3660, Cisco 3640 and Cisco 3620. Partitioning is also supported on the PCMCIA cards that support 8 MB or more capacity.
Two PCMCIA Flash cards are currently available: 8 MB and 16 MB.
Source
I'm not sure if anyone here as experimented with the PCMCIA slot on the 36xx series routers, but is any (non-Cisco branded) PCMCIA ATA Flash card compatible with the router? How about another type of memory such as a compact flash+pcmcia adapter? Will sizes >16MB work?