I can't seem to find a page where they'd put clearly the requirements for IOS upgrades. If I understood correctly, each Cisco device can upgrade to a higher IOS version, depending on the amount of DRAM and Flash installed. My first question is, are you able to say that to install IOS 12.4, you need X amound of DRAM and Y amount of Flash for any Cisco device? Or, does the memory requirement vary for each specific device? If it is the former, I can't a page that list those requirements. If it is the later, I can't easily find a page for each device that list the requirement for the updates.
All of the requirements can be found in the Feature Navigator pages.
"are you able to say that to install IOS 12.4, you need X amound of DRAM and Y amount of Flash for any Cisco device?"
No, you cannot just assume that across all the platforms. For example, for 12.4(24)T3 Advanced Enterprise Service
7200VXR NPE-G2 1024D/256F
7200VXR NPE-G1 512D/64F
All the memory information are listed in the software download page for their Flash and DRAM requirement.