A answer to the first question. There is no one set route for a flight to follow.In the real world you can not fly IFR, GPS or VOR to VOR unless you have a instrument rating. Each person must be certifed by an FAA pilot for a instrument rating for each type of plane. Since flying these you must follow specifc patterns & understand persision approaches.
The second question
Flights are assigned altitude according to which way you are heading. each direction flys certain altitudes
Flying north or west you would fly ODD altitudes
while EAST or South you would fly even altitude Of course I always get these mixed up & probably have the directions mixed up But it goes something like that.
The reason for this is to avoid any two aircraft at the same altitudes when flying in opposing direction ATC will try to maintain a certain degree of seperation between aircraft in the same airspace. So in realty there is no one set altitude a plane will use but there is a range and one can always request a different altitude from ATC either before the flight or in mid flight.

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You'd get a high alt. jetway. In Europe the airspace (especially around the southern UK) is so heavily trafficed that it would be risky to stray away from the airway system. But you'd always fly high alt. airways with a 737, regardless where you are in the world  

The cruise alt is determined by the aircrafts gross weight, route length and what kind of Cost index you'll put in the FMC. But european hops can generally be between FL310 to FL390. Longer routes would mean a lesser altitude since the aircraft is too heavy (with fuel) to cruise on high altitudes.

ATM I'm doing a flight between paris and Nice, And the FMC suggested FL390 as the most economical cruise altitude.

Gary is on to something though, he's talking about the Semi-circular rule, here's how it looks

http://www.ivao.org/training/tutorials/Ipack/Files/L7-RVSM-MNPS.htm

In Europe we have RVSM (reduced vertical separation mimimums) so it looks kinda different here. (hence the modifications)