You can't much upgrade the CPU while keeping the motherboard, as it's on an AM2 socket. If you were to upgrade the Graphics or CPU, you'd need to upgrade the Power Supply. If you upgraded the motherboard+cpu+power supply, you'd need to upgrade the RAM, as it's DDR2, but you can't upgrade the RAM without upgrading the Motherboard.
Basically - no. You might as well build a new PC.