i like canon but it's all personal preference. no matter which way you go it's the same question. cheaper new camera or used sicker camera, personally i would go with the sicker used camera if you think you are going to get into it.
if you opt for the canon route, think about a 30d maybe or a 40d. expect to be spending ~700 for a good body here are some good choices
and bhphotovideo will take your nikon point and shoot in trade too, they wont give you much for it but it's money you wont have to spend on this camera