For what it's worth, I don't agree with the 3yr thing. There is nothing wrong with a three year gap, but i disagree that it is a perfect number - it really depends on the individual family. When they are younger, they have less formed opinions about things, and from that perspective and be easier to integrate changes to (and things do change).
I completely agree with Misty. It is really up to the individual family. My DD and DS are two years apart (birthdays are 1 day apart). It has really been the perfect gap. My DD was always advanced. When DS was born she could have a conversation without without fault, and was completely toilet trained. Made my life so easy with a newborn. My DS is now 16 months old, and I am beginning to think about a third baby, however am worried about how he (and we) will handle the change, because he is still very much a baby. It depends on you, and when you are ready.

As for the relationship - with DD my husband and I suffered a lot. We found it really tough, and I honestly thought we wouldn't make it. Having a baby is such a huge adjustment (as you already know). With DS, our relationship has strengthened like I can't believe. We both knew what we were in for this time. No fighting... it has really been so so easy.