Insufficient memory refers to internal memory - it doesn't matter if you have an empty 128GB SD card, if the internal storage is full you have insufficient memory.
Rooting has nothing to do with whether you have sufficient memory to download a file. (If your free storage is so small that a few hundred k will make a difference, you're doing something wrong, and that's about all rooting takes.)
With 8GB of internal storage, about half of it taken by the system, you can't have too many apps installed, and none of your media files (music, pictures, videos, ringtones, alarms, notification sounds - even documents) should be in internal storage.
