Your single 1.8 G5 tower has a known logic board issue and Apple SHOULD repair it under warranty period.
They've already extended a recall for the 1.8 GHz Rev A iMacs because of
a bad batch of capacitors on those midplane boards.
Apple builds great products for the most part, but sometimes 3rd party suppliers cause these kinds of headaches.
If Apple won't repair it, then it's probably best for you to cut your losses
and sell it on eBay for whatever you can get.
Your X800 GPU should have no problem transferring into a dual 2.3
I'm not sure about your RAM, but the charts at OWC and Crucial
should give you that information.
I would get the CPU issue taken care of as a first priority then
work on a display.
Displays will continue to drop in price and improve, so there's no hurry.
If the G5 update Rumors are accurate, you could see a very favorable price drop on the existing dual 2.3 GHz models.
The best situation would be if your dad can talk Apple into accepting a return on your 1.8 providing that you are paying the difference for
a 2.3 even if it's a refurb.
Give it a shot anyways.
Good luck