The FIDs for the songs are unique across the two drives assuming (they come from the same empeg of course). The playlist structure is all stored as FIDs too.

So simply merging them all on to one disk is a simple case of moving them over. The player software looks on both disks so you can happily move them back and forth between drives without issue.

I'd install the 250GB disk and get it up and running, perhaps installing a song or two to check. Then I'd have the first drive as a slave and move everything over. Then the remove the first drive, install the second and do the same thing. There shouldn't be any clashes. Then rebuild the database and you should be set to go.

Play counts and ratings are stored in the dynamic data partition of the first drive. As long as the new drive has dynamic data partition that is the same or bigger a simple "cp /dev/hdc? /dev/hda?" where the question mark is the appropriate partition number should preserve all of that.

There is no tool that I've seen for manipulating the dynamic data directly. Not to say it can't be done. I think we have all the structure information. Not an insignificant job to create a GUI to modify it though.