If the system has been running fine in the past with MSFT Office or other fairly memory-hog apps, you should be OK with FS9. But, as was pointed out, 256MB is pretty small memory. It's possible that part of this SDRAM is bad so when you take all the memory with FS9 running, it faults. (We just saw this problem here at work on a brand new P4 system)
Go buy another memory stick, they're cheep and swap it for the one in your system. I bet the problem goes away....
