I would say, stick with gimp. It will make your transition to Photoshop CS much easier. Seeing that most of the basic elements are the same. Gimp can be a pain to learn, but if you want to start making sigs then it would be a great starting point for people with no cash in there pockets.
Yep, I have the GIMP right now, and its really user-friendly, not to mention that it runs quite nicely on my piece of crap (computer). I'm having trouble getting new backgrounds, though. If anybody can help me out, I'm getting really tired of "marble" and "pine". But, you should at least try Gimp out.