My first comment would your upgrading an old computer with a new one - the only bits staying seemingly are the case and floppy- So throwaway the case ,Floppy and the rest of it and get a nice new one -ready made and far cheaper than you can build.
However many of the volume makes are making computers now that are progressively 'upgrade proof'
IE you you can't or only ever so slightly.
So take a look at people like click here
they sell MoBo/CPU bundles to complete[upgradable] boxes.