1 old (Snow Leopard) iMac & 2 newer (Mountain Lion) MacBooks -- iCloud or DropBox? I have an old iMac (circa 2006, Intel Core Duo) which is running Snow Leopard (somewhat slowly) and cannot be upgraded to Mountain Lion, so there's no iCloud on that machine.

I also have 2 MacBook Pros which will both be running Mountain Lion very shortly.

My question is, I'd like to keep my "Documents" folder in sync across all of these three machines.  If I used iCloud to sync both MacBooks, is there an easy way to sync the iMac (minus iCloud) as well?

Or should I just use DropBox?  Any suggestions are greatly appreciated!
I think in your case Dropbox is the solution to keep synchronized your files.

You can continue to use iCloud on your other Mac with Moutain Lion (to synchronize your reminders or other contents that use iCloud).

Moreover, my understanding is that iCloud don't sync your Documents folder, so anyway for your need, you should use Dropbox.

However, be careful when you put your personal data online !