First of all, you're never going to have decent performance with that system. It was weak when it was new, and that was years ago.
As far as what you CAN do to increase performance, based on what you WANT to do, I'd highly recommend FIRST installing another 8GB of memory. There are no guarantees of compatibility when adding to existing memory, and it's always a much better idea to buy memory in matched sets, together, tested at the factory to be compatible, but you can increase the probability of compatibility by buying a second module that is the exact same part number, with the exact same timings and voltage requirements, as the one you have already.
You have a very low end graphics card. I would highly recommend looking at...