For the ATmega8 you can select the board "Arduino NG or older w/ ATmega8". To burn the bootloader you will need an ISP device or use the Arduino UNO as an ISP. You can get the USBasp ISP device on eBay for under $4.
You can use some other AVR processors by downloading a third-party 'core':
http://playground.arduino.cc/Main/ArduinoOnOtherAtmelChips