You can overclock some parts ( gpu, ram, CPU ), buy new parts other than ram, such as GPU, CPU, sometimes even buyin new powersupply or better cooling helps.
If the system doesnt have enough RAM, just add more RAM. Its one of the simplest things to upgrade, as the RAM sticks just pop in. One problem you may run into is many systems only have 2 RAM slots and you may find them both occupied. You may have to remove your existing RAM sticks and replace them with higher capacity sticks.