Thanks very much for the input - I'll look into the 500s. One other question - how crucial is the meeting rail reinforcement? I ask because one of my double hungs is very large and I don't want sagging to be a problem down the road. It's 50"+ wide.
+1 to WoW's comments. The 500 is a very stout extrusion as evidenced by its DP50 rating (without reinforcement). We've actually gone over 50" wide on them with no issues. That said, the reinforcement (800 series) is never a bad idea if for no other reason than peace of mind.