2x4 trusses and 5/8 drywall ceiling
The weight of the foam and furring strips is negligible. I'm not sure why you installed the furring strips unless you're concerned that the drywall will pull in when you screw it on, but they don't weigh much. 

Anyway, the weight probably isn't a concern. Use lightweight no-sag half-inch drywall if you have reason to think that the ceiling joists were taxed originally, but the additional weight of an eighth of an inch of gypsum won't likely break the camel's back.