Yes, all four are no longer produced - the 2600 series was superceded by the 2800 series.
The 3600s were replaced by 3700s, which have also been replaced by 3800s.
Your 2620XM has two DRAM slots, with a total maximum memory of 256 MB (each slot having 128 MB DRAM in it), though you will need to install a BOOT-2600XM-256 module if one is not already installed in order for the router to recognise 128 MB DRAM modules. The 2620XM has 16 MB Flash built-in, and you can add another 32 MB to it. The part number for the RAM is MEM2600XM-128D, and the part number for the flash is MEM2600XM-32FS.
The 2610 is very limited; its maximum RAM is only 64 MB. If you have less than that and want to upgrade, you need 2x MEM2600-32D. As for Flash, the router comes with 16 MB, which is the maximum. No Flash upgrade is possible.
The 2620 also has the same RAM limitation as the 2610. You can upgrade the Flash up to 32 MB, though you need to install a MEM2620-32FSBoot module first.
The 3640's maximum Flash is 32 MB. It will accept two 16 MB flash modules. As for RAM - that's a bit confusing; you can get the router to run in 64-bit mode by installing 4x matching 32 MB DRAM modules, giving you a total of 128 MB. If you don't install this configuration of RAM, the router will only allow you to have 56 MB of RAM total, and will run in 32-bit mode.
Hope this helps!
Kind regards
Graeme
