More thoughts:

(1) Just saw the post about water. This advice is way overblown. While I certainly would not touch/drink Nile water, anything out of a tap is so heavily chlorniated that it wouldn't harm a flea. Seriously: I have seen many, many people avoid water like the plague (!), only to get sick anyway. My advice? If you're there for only 3 days, don't eat any fresh/raw fruits/vegetables of any kind unless you peel them yourself. Drink mineral water if you wish, but no need to tape your mouth shut in the shower.

(2) Have you already made train arrangements? This is one of those areas where doing it on your own will save you $$, but unless you read and speak Arabic, I would not do this: The people at the train station - which is a total zoo, by the way - are notorious for selling the wrong thing, which you find out the hard way. Go to a major travel agent in the hotel you're staying at (and if there isn't one, go to the Nile Hilton) and have them arrange to get the tickets. Yes, it will cost more... But do you want your vacation runined by missing the train? Also: Make sure you get 1st class (definitely worth the small premium). Oh, and you'll have to arrange for your return as soon as you get to Luxor (not computerized yet, I'm afraid). You don't want to get stranded...

(3) Let me reiterate: Thomas Cook, which has an office at the Winter Palace Hotel, is an excellent place to arrange a car. A few years ago, it cost around $50 for a car and driver for the day. He wasn't a guide, though, so if you want someone to show you around it might cost more. Again, you could save money by haggling with a cabbie, but I advise against it if time is short.

(4) Personally, I think it's insane to do Luxor in a day, but so be it. Make absolutely sure you bring food with you. Water is obtainable, but unless you bring power bars, etc., you might find yourself stuck and starving. There's a horribly overpriced cafeteria near the entrance to the Valley of the Kings, but given how short your time is, you'll want to have extra calories at hand. (Water is pretty easy to find.)

(5) FYI, there is no way you'll be able to fit the Egyptian Museum, Islamic Cairo, Khan el Khalili, and the Cairo Tower in one day. Simply impossible. You'll quickly find that while Cairo is small, getting around is simply a lot more time consuming than in a western city; you can't zip around. My advice? Skip the Cairo Tower. Do the Egypt Museum highlights only (i.e., Tut), head to the Khan, where you can grab a bite, then wander until you can take it anymore... Not sure how you'll fit in a felucca on top of eating in there somewhere, but you seem pretty ambitious.

Good luck!