i'd stick with OS9. heat from harddrive will slowly kill the harddrive if it is really hot. Also i would install some programs that will monitor the hardrdive temperature.
if it is over 55C, then it should raise some concerns but nothing too much to worry about cause temperature concerns are subjective. some people dont' really care about the temperature of the hardrdive unless it is over 60-70C.
I'm not a mac user for a long time, only recently bought a G4 ibook so i'm not an expert or anything. However, i did deal with some harddrive troubles in my old laptop. I researched and ithink this really applies to your case:
basically, OSX is a big operating system and requires more memory (ram). when you access something that requires more memory than what you have already used, the old memory has to be deleted in order to make space. however, if you want it back, the harddrive has to access it again.
for those who have sufficient memory, the harddrive doesn't need to be spun as the old memory is kept in its place. so therefore, harddrive doesn't need to do any work at all.
Hence, i think u need more ram