your 250GB will be formatted again once it's in your slim unfortunately, anything on that will be lost.
after your saves are in the PSN+ cloud though you can get them onto your "new" HDD as long as the primary user account on the PS3 is linked to your PSN+ account where your saves are. it's then just a case of sucking your saves back from the cloud to your PS3. the software versions of the games in most cases will have to match with your saves so you'll probably have to patch your games back up to date for the saves to work and sometimes trophies too.
That's the way I understand (and have used) this
note that some games will allow you to export your games to another attached drive, but some games will not allow you to do this.
AFAIK, all games saves can be backed up to the cloud and restored from there (need verification) with PSN+
Proud Sony Rear Admiral
