A) Do I need to get 2 Arduino boards to be able to do that? (I think not)
No, just one, to which the ArduinoISP sketch will be flashed.
The blank/virgin (or even non-blank) Atmega328P will be in the breadboard.
B) Do I need an external clock crystal? (B1)if so...Why? ) (B2)any disadvantage (i.e. different result) if I dont have that?)
I believe you need them. The way I understand, before flashing the bootloader, you need to set the fuses (the IDE does this for you), and the fuses specifically ask for an external crystal or oscillator. Even if you pass this first step, from that moment on, you'll need the crystal to flash the bootloader because the uC has been programmed to need it.
Besides the crystal, you'll also need 2 x 22pF ceramic capacitors. All the three together should not cost more than 50 cents. Even in Brazil - where I live - this is very, very cheap.
C) The site linked says:
Note that these techniques only work with the Arduino Duemilanove w/ an ATmega328, not the Arduino Uno (or older Arduino boards w/ an ATmega168).
But (confusion?) continues with explaining about the Uno board? => is the uno(rev3) or is it not possible to be done with it?
Forget about that. Just flash the Arduino UNO bootloader to your Atmega328P.
Let me tell a story:
I got 5 blank Atmega328 (non-P, actually), and I followed the tutorial and flashed the Duemilanove bootloader to them. Last night I was working on a project-specific board I made, using it as an Arduino, but I had the UNO selected in the IDE. However there was no way I could upload any sketch to it with an FTDI basic attached to the serial port in my custom board. I checked my project entirely, both in DipTrace and in the physical board. Everything seemed ok.
then I remembered to change the board in the IDE to the Duemilanove, and voilá! My custom board started receiving uploads. I took my cheap chinese USBasp-close, flashed the UNO bootloader to my custom board (it also has an ICSP header) and it started working as an UNO in the IDE.
The greatest advantage of flashing your blank 328 as an UNO is that you don't need to remember what board you are using at the moment.
I strongly advise you to get a xtal and a couple of 22pF ceramic caps.
About buying bootloaded Atmega328P: I am about to buy a few from www.baligena.com
(found them on Amazon)