Whatever you do, don't let Marseille's rough reputation put you off going there for football. Their fans are generally very friendly, and the atmosphere they create in the ground has to be seen to be believed.

Getting there
Most of the flight options are to Nice, up to 3 hours by train from Marseille. Nimes is much closer, although the train transfer can be anything from 50 minutes to 3 hours depending on time of day.
easyJet (from various places to Nice)
Jet2 (from Leeds-Bradford to Nice)
Ryanair (from Stansted to Nimes)
BMI baby (from various places to Nice)

Places to stay in Nimes
Last time we played Marseille we flew to Nimes and spent the first night there. The hotel we stayed in was the Acanthe du Temple Hôtel, which was pretty central (map) and very friendly. To get there from the station you turn right out of the main entrance, then turn left up Rue Roussy after about 100 yards. Go right to the end of this street until you come to a main road with a sort of square opposite. Cross over into the square, then turn immediately right. The hotel is about 50 yards in front of you. Rooms were €54 for a double, €60 for a twin and €66 for a triple, including breakfast. Other hotels can be booked using the link below:
Hotel booking

Places to stay in Marseille
Etap hotels (cheap and cheerful chain)
Ibis (more upmarket chain)
Hotel booking

Guides, etc
Ryanair guide to Nimes
Rough Guides (select France, then Marseille or Nimes)
easyJet (special April 2004 feature here)
Tourist Office (including hotel list)

Club/ground
The ground is a bit out from the centre, but it's easy enough to get to on the underground. You get off at Rond-Point du Prado on line 2 (route map). There are also buses - the 19, 21, 41 and 83 are apparently the ones you can catch.
Map
Official club site
Ultras (official fans organisation responsible for travel, and probably for choreographing the fans in the ground)