The ATMega328-PU is an 8-Bit processor in 28 pin DIP package. Its like the ATmega168, with double the flash space, 32K of program space, 23 I/O lines, 6 of which are channels for the 10-bit ADC. It has a 1.8V to 5V operating voltage and runs up to 20 MHz with an external crystal.
Just download the Update above - extract the contents to your arduino folder, then run update.
This will add the 328PU to your menu & avrdude (the program the IDE uses to upload the sketches)