Ok experiance time!
I had a Compaq presario it blew up the CPU!
p4 1.8 ghz, 1GB Ram (upgrade from std 256 Ram) Geforce 2 graphics, CD rewriter, DVD drive (replaced with a DVD rewriter) and A (floppy) drive. 120Gb hardrive (was a 20GB but that failed in 2 years ago)
It was repaired but might as well be a new machine as the only Compaq componants left is the A-Drive and CDRom Drive. Still my 5 year warrenty says its still a Compaq pressario
What was replaced was the CPU (well I blew it up) the Motherboard (because the new CPU was not compatible) RAM (no longer compatible) and of course the Case because it no longer fitted the Motherboard

and a copy of XP as mine was a factory version.
The only thing that was holding back my PC was the Graphics card which has now been replace with a A6600GT.
It cost me $40 to have assembled
Now its
2.8gh, 1 Gb Ram, A6600GT Graphics, 120GB harddrive plus DVD rewriter, CD rewriter, Floppy A drive.
Chances are it will work out cheaper for you to get a Decent Motherboard, CPU,case with decent power supply and if your current PC is under two years be able to use that RAM and or upgrade to more RAM, and a Graphics card for under $1000 assembled. Using parts from your old PC.
In fact I have used the Ram for my old Compaq to upgrade a couple of dinosaur celeron machines I got from my employer from 16mb Ram (yes you heard) to 256mb. Add to that the 4 so called none functioning Phillips CRT monitors which you can pick up for around $100 if you want new.
Also as you have a PC then you already have a Keyboard, Mouse and Monitor you can still use. Keyboards/Mice dont need to be expensive to be functional.
Some more to think about?
btw, which part of NZ are you if Wanganui near I can point you to the guy that built my PC.