Cisco routers and switches are like any other piece of computing hardware. It needs slow long-term storage and fast short-term storage. In the case of a Cisco 2821 router this equates to compact flash storage for the long slow storage and regular RAM for the fast short-term storage.
Typically, people think if I am going to add RAM to a Cisco router then I will need Cisco brand RAM. While the logic is sound it misses a standard point. The RAM in a Cisco router is the same RAM they sell off of eBay or for HP, IBM or Dell computers. You don’t have to purchase the Cisco brand.
Recently, we priced out some RAM for a Cisco 2821 for a 1 GB strip of the generic stuff it was just over 100 dolllars. The Cisco RAM was about 3,000 dollars for basically the same thing.
The moral of the story, make sure you know what your buying.