I'm guessing that you mean the June 2004 G5. The Late 2004 model is a single processor 1.8 GHz.
The Early 2005 dual 2GHz will only go to 4GB maximum.
There's not huge differences between those two models that you mentioned. The largest changes come later on in the late 2005 models. All the G5s are fairly quiet, unless there's something wrong. Keeping the air flow and fan areas clean is a good way to help keep the noise down.