The way I see it, your PC is plenty good for your mom and then some. You don't really have to spend anything on it as of now. You should really save yourself a lot of money by just upgrading the gfx card and adding a further 2GB of RAM (4GB overall) and that too, if you are actually into gaming and not just a little bit of fiddling around - "mild gaming" as you say.
If that is the case, you REALLY don't need to do anything. If you feel your PC has slowed down a bit, just back up all your files and re-install the OS. It will be good as new in no time at all.