Presuming that the ATI chipset is using some of the system RAM, that doesn't allow you to use the full 2GB for the OS, so while you would see SOME performance improvement by upgrading to 4GB, I don't really think it would be impressive. If there was a way to buy the RAM, try it out, and then return it if not satisfied, I would use that approach.
I have installed w10 on an even older/slower pc than yours and it's quite enjoyable. In fact, it may leave more available resources than Win7. Make a backup copy of your operating system partition and do a win10 CLEAN installation. Install the w7 drivers for your videocard and test how it works. What browser are you using? some browsers like chrome make it hard to have a good experience in Youtube for older cpus/vgas. Just check Youtube on Edge for instance.
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