Alpe d'Huez is, from my experience, a good intermediate resort (blues and reds), Les Arcs is okay as well. Because of the huge number of resorts in France, the best thing to do would be to have a look at resort ratings online - http://www.skiclub.co.uk/skiclub/direct/resorts/de fault.asp
(this has info taken from the 'Which' skiing guide). Then all you have to do is trawl through the holiday sites (eg: http://www.igluski.com/
) to find a good deal!
Stay in the Eldorador, in La Plagne. Belle Plagne is in the same area, and if you go for a week you will not exhaoust the ski-ing. Plus, you get a day in Val Disere on the La Plagne lift pass. Most of the accomodation is self catering, Eldorador is one of the only hotels. We have skied there 3 times.