The only way its possible is if you get Logicboard and all the CPUs from a Quad G5 and transplanted it. Also The Quad had a liquid cooling system. I think your Dual 2.5Ghz does also, but I can't remember.
Even if they do, you would have to transplant that as well. In short, it can not be done without spending a ridiculous amount of money.
The G5 Logicboard firmware are turned for a specific processor, they are not interchangeable.
-iGrant