The lower the ISO, the better the quality, but the more light you need. 400 ISO is for amateurs (your Mom and her point and shoot). If you want to take realy good portrait shots, get good lighting, and use 100, or even 72.
On the other hand. Some people really like the grainy look (it only looks good in B&W in my opinion). So if you wnat a grainy look, go with an even higher ISO 800, or 1000.
Bt make sure, if you want yur portraits to turn out awesome, that you use a diffuse filter.
No one on their death bed has ever said 'I wish I had played it safe'