I haven't gotten the whole process figured out, but it seems like I am out of luck...
I currently have some 2621xm routers with this IOS image:
It is the feature set for IP Plus IPSEC 3DES. I use this for connecting to the router via SSH. I would like to upgrade to 12.4 something, but it seems like everything requires 128 MB of DRAM.
Am I looking at things wrong? Any help is greatly appreciated!