Ahead of you on this one, gwyddn. The motherboard in the Optiplex is custom - the standoffs are not standard. The power supply (at least in the one I have) not only is complete crap, it is non-standard in size. this would be very hard to work with and not worth the trouble. I got mine for free, upgraded it with memory from another machine and added an AGP graphics card for $15. It does fine for DDO.
I'm officially scrapping that idea! Thanks for saving me the $$ and the headaches of having tried and failed.