200% is the maximum in Fallout 1, and 300% is the maximum in Fallout 2.
Non-combat skills don't really need to be raised above 100%. There are a very few cases where a non-combat skill will need to be raised above 100% (like building Skynet in Fallout 2), but 99.9% of the time they don't need to be above 100%.
Combat skills only need to be above 100% if you plan on use targeted shots. If you use burst weapons, then going above 100% isn't really going to do much. I don't know the exact formula behind combat, but there are a lot of negative penalties that really add up.
Armor Class of the enemy is a penalty to you. Distance adds penalties. Night time and poor lighting conditions even adds penalties. If something is between you and your target, this adds penalties as well. Targeted shots adds huge penalties. I know targeted shots to the eyes is a 60% penalty.
So even if you have 100% in a weapon, your actual percentage chance to hit may be very low due to all the penalties. This is the reason to boost combat skills above 100%. Once you get to 95% accuracy (100% is always impossible due to critical failures) at targeting eyes from long range, then you don't need to raise the skill anymore.
Like I said previously though, if you plan on using burst weapons then the skill doesn't need to be raised as much. Burst weapons are best at close range, so distance penalties don't apply as much. You also don't get the severe penalties for targeting specific body parts.