Just spent 7 months working in Sudan.
As a Christian things are tolerated slightly in Khartoum, but anywhere outside then covering up is a must.
Sudan has some of the friendliest people i have ever met. When you get to Khartoum look up the Acropole Hotel (Greek run) which is a bit of a cult place now for travellers, the owner George will make anything you need happen and is one of the nicest guys you will ever meet.
There are a few nicer places to stay, but the service and help are just too good, when i was there last many women stay there from the UN, BBC, and various aid agencies.