Hello,
If you want a more responsive system, go for an SSD.
If you want a more powerful system than the stock one, try to find the exact model for your CPU (install CPU-Z, it will give you the infos) and see if there is something more powerful than your actual CPU:
http://www.cpu-world.com/Sockets/Socket_FS1.html.
Just remember that a more powerful CPU could not work properly because of the thermal constraints of the cooling system and the power constraints of the motherboard or the AC adapter.