You could start a rebuild with phenom II, mobo and ram and reuse the rest. There are not a lot of improvements you can do to the current setup that would not be CPU limited.
I think your assessment about the limitations of the cpu, mobo and ram are right. I have been put off by the cost of an Intel 930, Asus P6x58D Premium S1366 ATX Motherboard, and 6G Corsair XMS Performance Memory that I have been eying. Now I am looking into the Phenom II and it would be considerably more affordable.