New progress!!
Put everything in, hard drive caddy, DVDRW and all, all fits perfectly!
Took it back out again to test with a old worthless drive (didn't want it blowing up with my current drives in there)
Success in turning the machine on, although it did keep saying keyboard error or no keyboard attached (EDIT: a USB keyboard was attached, I'm not that stupid), which was odd, plugged in a old PS2 keyboard and it worked fine, reset the bios back to fail safe defaults and tested installing XP.
The machine works well, I got given a PNY(??) Nvidia 8800GT 256mb graphics card, XP doesn't have the propper drivers installed so it's very very slow at the moment, but it looks promising
Pictures showing various stages of progress as follows: