It's really your choice to use a sound card or not. You don't need one. You can use an external sound card or USB Dac. I would opt for the external conversion which ever way you go.
There are many factors here when choosing a PC. Do you want it for HT as well? Or is this for just 2 channel music. Do you want to go wireless or hard wired?
Yes it would be wise to back up your HD.
You could go PC> USB Dac> Home stereo
or PC> internal or external sound card> home stereo
or PC> wireless router > wireless receiver> Home stereo
The possibilities seem endless.
There are many factors here when choosing a PC. Do you want it for HT as well? Or is this for just 2 channel music. Do you want to go wireless or hard wired?
Yes it would be wise to back up your HD.
You could go PC> USB Dac> Home stereo
or PC> internal or external sound card> home stereo
or PC> wireless router > wireless receiver> Home stereo
The possibilities seem endless.