A good start point is the key...
Resolution can be a confusing topic. However, if you do not have a good start point you will be doing well to achieve a good final result. After all, you can't 'add' more detail later, you can only make sure it is there in the first place.
We can take your image as a digital file, a 35mm film negative, a film slide or an actual print, although this does have it's limitations.
So, regardless of whether you have digital SLR shooting in RAW, or an old faded photo the attention to detail starts here.