As fmw said, if you have Paint Shop Pro, you have just about all you need. I have Paint shop 9 and it has more than I really need. I am playing with the trial of Paint shop XI and will probably upgrade to that. Either version will do all the curves and levels and layers and masks and what not you hear talked about in the Photoshop places. Many of the Photoshop plugins for special effects will also work, i.e. Power Retouche plugins. Many tutorials on Corels website also for some extra help.
Have fun with it.