If you're upgrading to that extent, it's probably worth checking out the price of a completely new machine built to your own specs and keeping the second one as a server/MP3 jukebox etc., or donating it to charity.
I have had a few PCs. My upgrades have consisted mainly of adding memory and better graphics card. I've only ever once upgraded the processor and that was from a Cyrix to a Pentium 233MMX
