I started with the Bareduino328, which was very easy to assemble thanks to their great guide.
The only drawback of Bareduino is that the shipping rate is high when buing via eBay:
the Bareduino328 price is 5.5 Euro.
shipping to Slovakia is 7.4 Euro.
Later I discovered this kit on Ebay ATMEGA328 - PU - ARDUINO UNO KIT, which has better price:
Atmega328 Arduino Uno Kit: 5.5 Euro.
Shipping to Slovakia: 1.8 Euro.
I advise you to start with the Bareduino boards, it is slightly more expensive approach, but they did a great work and their online documentation is very useful.
Both kits provide Atmega boards with Arduino bootloader.
The Bareduino328 includes the ATMEGA328P-PU MCU, the second (cheaper) link ships ATMEGA328-PU. The main difference between these two chips is behaviour in low-power mode. More info here.
All my experiments below are based on the (older) ATMEGA328-PU.