I agree with Kroc in as far as GIMP is not as capable as Photoshop, but that it will be plenty for you to do some really good stuff. GIMP has a quirky interface, though I hear Photoshop isn't much better in that regard (don't know myself, as I can't afford any Adobe pay software).
But yeah, GIMP will do plenty. The ONLY way I would spring $800US for Photoshop is if I could use it to make that money back. As it stands, I can make money with GIMP. Then again, I don't do it professionally... but then, you don't plan to either, so... ;)
I don't have a feature comparison chart or anything, though, so there might be some earth-shattering, gotta-have-it feature I may be missing. I highly doubt it, though.