I noticed a big step in speed going from a G4 1.25 MDD to G5 1.8...and both these macs are single processors. Now have a dual G5 2.0 which is even faster. If you have enuff cash to get a dual, go for it. I am sure you already now how to eek power out of your system, as you kept that G4 for a while, which means youll keep this new one for awhile too. My average is new mac, new to me at least!, every 3 years. YMMV.
My advice, get the fastest Mac you can afford, even if has less ram/HD space, these can be upgraded later.
How much difference is there between a G5 1.8 and 2.0Ghz?
I see that they 1.8's go almost 400 dollars cheaper. Would I be losing alot of power?
And how does a 1.8 G5 compare to my 1.5Ghz G4 Is it night and day past what I have now?
If the 1.8 is $400 cheaper, you might be looking at the single processor 1.8. There was also a dual 1.8, whereas I believe the 2.0 and above were always dual processor or more in the G5 series.
It is a decent jump going from a G4 1.5 to an early G5 (not so sure about the single processor G5 though), however, my new core duo 2.4 macbook pro smokes my dual 1.8 G5 desktop machine in most regards, and is much, much quieter.