Memory chips from the different Mnf. and speed may be incompatable. Not a big iissue with XP but under load system may not respond correctly.
This application gives information on make, model, and serial number of hardware attached to your system
System information is under All Programs\Accessiories\System Tools
In tool bar, under Tools, open DirextX diag. The 1st window will list your cpu speed and the amount of memory you have and using. Good for win9x as XP has the task mng for memory usage.