"Readyboost" will give little to no advantage on most cases, and, for a very slow CPU it will just create more work for it, resulting in CPU overload, also, even USB3 is just so so much slower than RAM.
If you're unable to increase RAM, find an OS that use less RAM, not to mention the obvious again, I just say, as I write this, my OS is using less than 1GB of RAM (and 300MB of those are for Firefox browser) EDIT: with browser closed ~500MB RAM usage, because Firefox is set to also cache content to RAM.