Sorry I haven't used GIMP so can't help there with a comparison. I'm also a Nikon girl but I believe the 50D is a very good camera

Photoshop is a very powerful and complicated program but well worth it once you learn all the intricacies of how it works. You can download a free 30 day trial if you want to see how it works and if you have a library near by see if you can et your hands on a Scott Kelby book Photoshop for Digital Photographers (it will also have the version eg CS3 is the PS version I have but the current one is CS5 so look out for that).