I don't quite understand what you are proposing to do with RAM. Are you planning to add 2 sticks of 1 GB RAM (a total of 2 GB) to an existing 2 sticks of 512 MB RAM (total of 1 GB) for a grand total of 3 GB of RAM? In any case, I believe it's always advisable to use identical sticks of RAM or you may have weird issues.
But regardless of whether you have 2 or 3 or 4 GB of RAM, your build will not be able to play newer games because of the older CPU and GPU parts. Imho, I would not suggest spending really any money into this PC unless you are certain you will be able to recycle it into your next build.
For example, a decent SSD should work okay with this system even with its limitations (no TRIM, 3 GB/s vs 6 GB/s). You may not be able to make the most of it, but it'll still be super fast compared to a mechanical drive. Hope that helps!