[nitrous9200]

There is always some memory used even after hard resetting the phone, but the space usually comes from other programs that store files in the main device memory. I believe the default configuration on the stock ROM for Opera is to store cache files on the main memory, which will eat up space. It is also possible to delete some of the Samsung ringtones to free up some valuable space.
Flashing to a stock ROM would certainly help with storage space - most of them have at least 90MB free from a fresh flash, usually over 100MB - but you would lose some of the creature comforts (most Samsung customizations, the widgets, etc) in exchange for that space, as well as having the latest bleeding edge copy of Windows Mobile 6.5. If you're not keen on messing with your phone, I would stick with the stock ROM (at least update to the new CF03 software if you haven't already) and delete some of that extra fluff if possible. I'd be more specific on deleting things if I could remember, but some of it is on the forums. I believe the ringtones are located in the Windows folder as a bunch of MP3's.