It would be worth is to look into an upgrade to a new motherboard. The Phenom II Processor series are about 30% faster clock for clock over the Phenom series. I would look at a motherboard that uses DDR3 and can support a Phenom II 945 or a Phenom II 955 Processor.
If you do not want to spend that much you can always look at some quality solid state disks. A good solid state disk can make your system faster in some ways.