I would like digitalize all my photos and I bought the Canon CanoScan 9000F. I have no problem of space and hence want to scan the photos with the highest possible quality. For a printed photo, is 4800 dpi really better than 2400 dpi? Is it better to use ScanGear or VueScan? So my question is what are the correct scan settings (Unsharp Mask, Reduce Dust and Scratches, etc.) to achieve the best result?
Yes, higher DPI will give better results than lower DPI. The actual pigments that the photo exposes are very small (small enough that they are small dots at 500x zoom if they are photographically developed). Also, your sampling will not line up exactly with the original dots, so oversampling is beneficial even if you could exceed the effective resolution of the original image.
The application shouldn't matter, but what work you do in post production should matter. One of the programs may do a better job using automatic settings, but that is going to be situational and the best bet would be to try some photos and see which looks better to you.
As far as post production work (such as unsharp masks and dust and scratch reduction), it is highly, highly conditional on the image being scanned. You will have to use your judgement on what looks best to you. If the images have damage, then dust and scratch reduction may be able to heal some of it automatically and unsharp masks may make the image look crisper, but at the possible expense of some fine detail.
There really isn't a substitution for trial and error. There is a reason that the functions are options and not automatically on or off all the time.