I haven't tested this myself, but I've heard from users who have issues with sync if this gets to 4GB
With ACT! 10, you can attach a link to a file instead of movinf it to the supplemental files folder, so another option is to put the files on something like www.foldershare.com and link than to the contacts
I have been speak to someone about a tool to better handle large volumes of attachments, but it might not be able to work with sync... discussing it this week
There is a 4gb limit, but the error occurs when you try to unpack the database rather then save it. A workaround is to cut the database without attachments and then manually copy the files down. Note that backups of the main database will also fail to restore if you break the 4gb limit, but they will still "successfully" backup. As far as number of files, you'd have to be well over 200k files before you starting having serious performance issues.