Egypt travel (Luxor-Cairo)

Hi

Just out for a bit of advice and hopefully an idea of costs. I have
flights booked with my girlfriend to Luxor at the end of October and am
in the process of planning out my trip (no hotels booked yet). Having
never been to Egypt before, we obviously want to visit Cairo to see the
city and Pyramids etc. My initial thought is to spend 3 days in Cairo
and take an internal flight, I see on Expedia I can get a return flight
from Luxor to Cairo for roughly £100.

Would I be better off getting all this when in Luxor? I have heard that
there are plenty of places selling overnight and longer excurions to
Cairo from Luxor when out there but there never seems to be a mention
of how much these trips costs from Luxor travel companies (I hear
several mentions of 'Jollys' in Luxor, but never any prices for the
Cairo tours). Would these be more cost effective? Or would I be better
off booking everything online from the UK before I go? I guess I'm
partly aware that if planning/booking from here, I won't be double
booked having a hotel in Luxor but staying a few days in Cairo at the
same time, but on the other hand, I don't want to leave myself short
and be running around out there trying to find accomodation!

Hope thats not too long winded! Any advice would be greatly
appreciated!

Gavin