MatthewClement,
Hellinikon airport closed for commercial traffic about two months ago, so while the info you posted was accurate, it does not apply any more.

The new Athens airport is the Eleftherios Venizelos airport, and it is about 25 km from the city.

Taxi now takes, obviously, longer and it is more expensive. Expect around 4-5,000 GRD for the center of Athens during daytime. The taxi drivers are asking the government for a special raise for airport trips, because the consider it our of their way.
Taxis depart outside the Arrivals area, between exits 1 and 4.

There are three express bus lines to/from the airport:
E94, between the airoport and the first Athens Metro stop (Ethinki Amyna) on Mesogeion Ave. Hrs: 06:00-23:35, frequency, every 15 mins approx.
E95, between the airport and Syntagma Square, i.e. the center of Athens. Hrs: 24hrs a day, frequency, every 20 mins from 06:00-24:00, every 30 mins or more from 24:00-06:00 (detailed schedule available).
E96, between the airport and Peiraeus, via Glyfada. Hrs: 24hrs a day, frequency, every 20 mins from 06:00-24:00, every 40 mins or more from 24:00-06:00 (detailed schedule available).

The ticket for these buses is 1000 GRD and is valid for 24hrs. They depart from a point outside the Arrivals, between exits 4 and 5 (the Schengen exit). Here is a map, in greek, of their stops: http://ta-nea.dolnet.gr/D08032001/1kn19a.gif

There are six rental car agencies: Hertz, Budget, Europcar, AVis, Sixt and Alamo/National. Bear in mind that the toll road to the airport costs 300 GRD, to be increased further later this year.