Well, a new processor would help you run programs quicker. As will adding more memory to your system. however, considering the age of your system, you would have to;
1. Buy new Motherboard
2. Buy new CPU
3. Probably buy new memory
4. Would more than likely need to install a bigger power supply.
If all these tasks seem slightly daunting to you, i think you may find it easier to buy yourself a new PC.