Fundamentally there is no limit of the amount of data that can be searched, 80GB of PST files is quite a modest amount, but there is a limit on the amount of data that can be collected, ie the amount of results that can be stored in memory.
If you search for something very broad like:
Containing Text: is OR the OR and OR ed
Then you'll likely consume too much memory with the default settings. There are some steps you can take to reduce the memory consumed:
How do I avoid "Out of memory" issues when searching very large repositories
However, if you think it isn't a memory problem then please click the menu option Help->Generate Error Report and some one from Tech Support will be able to look into it.