If you want to do stuff like whitening teeth, healing scars, general portrait retouching in Photoshop then run, don't walk and get a copy of Katrin Eismann's brilliant book
Photoshop Restoration and Retouching. It's marvellous!
Also for retouching-type stuff you might want to check out
http://www.retouchpro.com -- a very friendly community of retouching professionals and amateurs -- mostly they use Photoshop but there are a fair few PSP users too so whichever option you end up going for you should get some great advice and tips there.