There's really no need to worry, the political situation is kind of unstable, but you won't even notice it if you don't go to Tahrir Square when there are protests (90% of the time even Tahrir is fine to hang out in). The worst you're likely to get is some overly pushy shop owners, especially if you're going to Luxor and Aswan. Like Crashdummy said, the Sinai is still dangerous at the moment, but you're not going to be in the worst areas if you're only going to Sharm or Dahab, there are lots of checkpoints along the way, and your bus/transportation should have some security on it. Aside from Sinai the country is pretty safe, just use common sense and you'll be fine.
The train to Aswan is safe, but make sure you get a first class ticket, because there's a HUGE difference between first class and the crammed and dirty second class. Also, try to buy your train ticket at least a week or so beforehand, because the overnight trains fill up really fast. I can speak from experience that showing up to the Cairo train station and asking for an overnight ticket to Luxor/Aswan for that night is not going to work.
I've been living in Cairo for a few months, so if you have any other questions I'd be happy to answer them.