Hi there!
Typically yes, it is possible to upgrade the amount of RAM and flash your 2600 routers have (unless they are already maxed out).
As for the upper limit, that depends on the model in question.
For example, the 2651XMs we use at work have a higher amount of RAM and Flash than my original 2651 can support.
If you can tell me what routers you have, I'll help you find the upper limit of RAM/Flash for each, and the part number for RAM and Flash for that router.
Kind regards
Graeme