Is it necessary to reinstall your OS after adding more RAM?
Original answer (about reinstalling)
No, you don't need to reinstall your operating system. With Windows XP / Vista it is possible that you need to reactivate (my experience is that about 1 out of 10 XP/Vista needed reactivation after RAM upgrade), however this should go better with Windows 7.
Address space limitations (maximum usable memory)
However there is some limitations on how OS handles RAM. Most notable is difference between 32-bit and 64-bit systems, there if you have 32-bit Windows XP installed then you are limited to 4GB usable RAM even if you have added more than 4GB of RAM. If you need more than 4GB for programs then you must upgrade OS either to 64-bit or to one that fully [no hard limits] supports PAE.
See also:
Notes:
- There's different versions of PAE (Physical Address Extension) supporting different sizes of virtual address spaces.
- 32-bit Windows XP can not use virtual address expansions offered by PAE (see MSDN hard limits article).
- Maximum address space and maximum program memory might be different (4GB/3,5GB for example).
You need to upgrade to 64-bit if you install more than 4GB of RAM.
Other than that you don't need to reinstall.
Note that you'll need to reboot the machine cleanly afterwards. You won't be able to just "resume" from hibernate.
No, OS reinstallation is not required after installing more RAM
Reinstallation is preferred for major hardware changes to avoid conflicts due to driver issues, but barring a reactivation which may be required if you changed more that your RAM, nothing else is needed