Honestly, I wouldn't do it. Unless you can get used DDR3 RAM very cheap, or nearly free, it's not worth putting any money into this aging platform. Most modern systems for the past 3 or so years have used DDR4 RAM, so whatever extra DDR3 RAM you buy will not be usable if you upgrade in the future.
You can mix and match, although sometimes it will give you trouble with mixed brands. I don't have experience with mixing RAM, though.
What are you using this system for?