HI Esick,
No ther are not compatible.
The 2610-2621 uses 100-pin EDO DRAM DIMMs
The 265x and the 2600XM use 100-pin SDRAM DIMMs which are NOT COMPATIBLE with the existing 2610-2621 DIMMs
Here you go with the link
http://www.cisco.com/warp/public/63/2600_architecture_23852.html
HTH
Ankur