I agree that the belt adds a nice finished look to a kilt.

Traditional kilts that are worn up near the ribs do need the wider belts as Hamish suggested. I'm not partail to the flashy buckles for casual wear either. It just looks over dressed.

P1M has a great "casual" buckle to fit a wider belt. I say casual, but that style buckle can, and does, work in a more formal setting also.

I think the casual kilts like are made by USAK can get away with a narrower belt. I have a 1.75" belt that works very well. Due to the shorter overall length of the kilt a 2" belt would just be too wide and throw the proportions off.