Hmm, I've never really tried OfficeDraw or Finger Paint Magic or Paper! (and I didn't know you could draw on GIMP :O), and hells yes Photoshop is veerrrryyy expensive, but personally I would go for something that can support layers and is perhaps more geared toward art in the sense that I've been using (y'know, tablet and pen). If you have the chance to try a tablet, it's real fun and smooth...takes some getting used to in the beginning but then it feels no different from drawing on paper later. I got used to mine within ten minutes or so.
MyPaint is free and it synchronizes with an art tablet and pen, but programs such as PaintTool SAI and Sketchbook Pro are much, much better. SAI costs about $58 and Pro is around...67$? $60? I forgot. But I want them one day. Or at least, SAI, since that's cheaper.