Default pagefile settings will be a file on C drive that can grow as required, so this should be fine for virtual memory.
8GB RAM should be enough.
Open up task manager
Click Processes
Click "Show Processes from all users"
Click View -> Select Columns...
Tick "Memory - Working Set"
Click OK
Click on the "Working Set (Memory)" to sort by this column.
Click twice if necessary so that the largest values are at the top.
Have a look at what processes are using a lot of memory.
If nothing is hogging your memory, it could have just been an issue with Fallout 3.
It may not happen again.