Is a surge suppresor w/ UL1449, 330V, and 905 Joules rating good enough to protect a 550W (Enermax EG651p-ve) system?
 

I am concerned about my current surge suppresor's ability to protect a
550W PSU (as well as the rest of the system). I have several
questions.

I read in MM's book that 800 Joules is the minimum amount of
protection for a system. Is 800 Joules suffiecient for any system?
No matter what the wattage?

Is 905 joules an insufficient amount of protection for my 550W power
supply? If so, why?

Is there some kind of mathematical formula I can use to calculate a
joule rating that will be sufficient for a 550W PSU? That is,
assuming there is some sort of proportional relation between joule
rating and PSU wattage. Such as a higher PSU wattage requires a
higher joule rating.

Is it worth the money to buy a UPS? I was thinking about buying one
but a UPS with the proper amount of wattage (for my system) will cost
around ~$150.