I wouldn't be so presumptuous as to recommend a lens but, if Gordon will forgive me for including an off-site link, I found the reviews at
http://www.the-digital-picture.com/Revi ... views.aspx
helpful in making my own choices about lenses that Camera Labs hasn't reviewed yet.
As you have a zoom which covers the 50mm you mention I am interested in your reasons for choosing this focal length. With the 1.6x multiplier of the 400D that gives a full frame equivalent of 80mm which some might argue is quite long for casual indoor work although it is a classic length for portraits.