|The first thought that should come to mind is: Why?|
Is the old PC too slow? Not able to do what you want it to do? You want to buy some new add-on that will not work with your old PC? The old PC is proving unreliable.
If the old PC is still doing what you want to do then perhaps you should stay with it. A client decided that Windows Vista was the "in" thing and bought a new PC with Windows Vista Home version installed. Within a month she came back to me and asked me to install Windows XP as some of her add-ons (scanner and printer) did not work with Vista and she was so used to the way XP worked that she found the Vista way just too confusing. She would have been better off with cleaning up her old PC (re-installing Windows XP does wonders for a clogged up, slow-running PC) and adding some more memory (XP runs much more smoothly with 512MB than 256MB)