Providing you are eating a well balanced nutritional diet there is actually no need for additional vitamin supplements, they are just a marketing ploy. 800mcg of folic acid is all you need
Of course if you have health issues, diet restrictions, or pregnancy complications like really bad MS you would benefit from taking one and are not going to do yourself or your baby any harm. Although make sure it is a pregnancy specific multivitamin as other normal ones can contains too much Vit A which can have a harmful effect on the baby.
eta - but in answer to your question: No you can't overdose on folate as it is water soluble so any excess you will just wee out

However in saying that excess folic acid can mask a VIt B12 deficiency so best just to stick with the 800mcg unless your MW or OB says otherwise.
eta part 2 - sorry for the spiel!
Edited by Mrs_B