If you are heading to Chamonix now Les Grands Montets(best and steepest terrain) will have the best conditions, the area is basically all off piste. Brevent and Flegere are best early in the season, they are south facing so the condtions aren't ideal for now. Les Grands Montets is north facing so the snow there is always fairly good. The thing to remember with Chamonix is that the areas aren't linked up. Ideally it is best to have a car as the bus service is quite poor.
To go to the very top of Les Grands Montets you have to pay extra (5euros), well worth doing as the snow is always the best up there. There are marked off pist runs from there be careful not to stray on your own there as there are crevasses around so make sure you are very careful.
If you are lucky enough to get a powder day there, get there early, the place gets tracked out very quickly. If temps stay cold enough head over to Le tour and ski off the Tete de Balme, when you get to the top of the chairlift hike up for 10-15mins to the left this will open up a bit more of an area for you, nice bowl that, very varied terrain off the tete de balme, and you can get tracks there a few days after, it is almost like you get the place to yourself.
Worth going to the top of the Aiguille de Midi where the Vallee Blanche is(not an amazing ski unless you do the non tourist route, must take a guide for that one or else you will definitely fall in a crevasse etc...), veiws are out of this world as you are at 3800m and the mont blanc is just behind you.
If you want anymore info let me know.
Have a good trip, I think they got some snow there yesterday, saw some pictures of Le Tour that looked good.http://www.chamonet.com/snow_report.php#