Go the vector route if you're going to do book covers as they are resolution independent and you can export any size to a raster format whenever you like. Rely on styled text more than photographic images as bitmap/raster images don't magnify well. Remember that if you're going to print, it's CMYK not RGB and you have to take that the colours you see on your screen aren't going to match the colours you will see on a printed cover (if you're using photoshop/illustrator this shouldn't be as much of a problem).
I personally prefer Inkscape as I'm running Linux and, well, it's free.
Here's the cover I've been working on today in Inkscape for my indie publishing venture just to give you a taste of what's possible with free tools. Remember that if you use vectors they are resolution independent, they will look just as good at postage stamp size all the way to billboard size.
Last edited by Dusty Bottoms; 05-02-2010 at 08:23 PM.