I have a very similar machine, and the upgrade possibilities aren't good at all. You can upgrade the cpu to a quadcore or a xeon 775 cpu
http://www.cpu-world.com/CPUs/Xeon/I...569KJ087N.html , if you look carefully you might still be able to find a spare Xeon CPU in a local store.
Upgrading the memory might be more dificult then you imagine, low entrance 775 motherboards don't suport more then 4gb, and you already have that memory, look in your motherboard manual, or try searching the pdf by searching the motherboard number in google.
Upgrading the GPU might be of no use, as you problably still using pci-E 1.1, which means you won't benefict from a faster GPU.
The best option is really buying a new machine, you can buy, or assemble one for 300€ or less if you keep your current periferics.
EDIT:
If you want to try a new CPU, i advise you to try a new PSU aswell, at least 600w.