You may just want to look at a new system and try and sell your old one for anything you can get.
To upgrade the CPU to a Dual-Core you will likely need to upgrade both the motherboard and ram. Once you upgrade the motherboard, the graphics card if you have one would also likely need to be replaced.
If you are currently using an on-board video card, you could just get a new system with a built-in video card, but likely you want a discrete graphics card for the activities you mentioned.
Really, there is not much beyond the HDD, if that, which is likely salvageable.