For 3D games, the video card usually makes the most impact - the 3870 is a little old, so try an HD5xxx series, like an HD5830, or if that is too expensive, an HD5750. Your cpu is also fairly low, though it should still be usable - check the task manager to see if it is at 100% when playing, if possible. There's no point trying to upgrade that unless you are willing to replace cpu, motherboard, and RAM, since I would recommend an i7 or i5 to replace it, and those require difference motherboards and RAM.
