In case anyone is looking to upgrade Cisco 2900 series routers with more memory, I can confirm that the following works in a 2901.
HP 2GB PC2-5300 CL5 DDR2-667 ECC UDIMM RAM Module - Part Number PC2-5300E-555-12-G0 - $22 from eBay/okmemory
Cisco CISCO2901/K9 (revision 1.0) with 2506752K/114688K bytes of memory.
Processor board ID FTX000000AB
2 Gigabit Ethernet interfaces
1 terminal line
1 Virtual Private Network (VPN) Module
DRAM configuration is 64 bits wide with parity enabled.
255K bytes of non-volatile configuration memory.
250880K bytes of ATA System CompactFlash 0 (Read/Write)