Actually, I think the best win would come from having OLD on the filesystem. CUR would be excellent, too. As it is, OLD has complete copies of every edit ever made, and it forces MySQL to consume an unholy amount of memory. :(
There's nothing magical about filesystems--the same work has to be done whether the data is in a database or a filesystem. However it is true that filesystems tend to be optimized for different kinds of access patterns than databases, and those access patterns may be better suited to big chunks of text.
Also, I don't see why the size of the old table has anything to do with the amount of memory used by MySQL. There's almost no difference in performance between a database with 10,000 entries and one with 100,000 entries. A more significant win is likely to be reducing the size of the /individual records/, perhaps by putting the full text in the filesystem and just having pointers and stats in the database.
Moving only the old data to the filesystem would also be less of a problem than moving the cur records, because we don't have the problem of losing MySQL's fulltext index, which we need to keep for the cur table to implement the search function.
Another win for the two-server setup might be keeping old text in the front-end file system rather than the database machine, to reduce traffic over the wire.