You should be able to drop an ATmega168 or ATmega8 (with appropriate bootloader) onto an Uno, with no other modifications (to the Uno.) You would have to either buy these chips with a bootloader pre-installed, or have some other mechanism for installing the bootloader.
In the "board" menu, you would need to select the board based on the chip and bootloader used, rather than using "Uno." So an Uno Board with atmega168 and the old bootloader would be called a "Diecimila ... with mega168"