But basically the atmel328 chip is the equivalent of the intel x86 chip?
Hmmn. An intel 8086 and all later variants will make copies of your stuff and leave it lying around in extra shift registers which can be useful for debugging and necessary for identification when on the internet. A lot of websites have been set up to do nothing until they have returns which were once upon a time mistakes or proprietary Intel circuits. Read up on the "pentium bug" by which certain big name processors not only got the wrong value for a maths calculation but were relied upon to always get the same wrong answer.
The atmel328 has no such nonsense about it.
Unlike intel chips, it has flash program memory, and volatile data memory on die, and does not require a return from an "are you sure" popup window because you can just switch it off.
The raspberry pi, in having a bump-bonded memory chip stacked over its processor, is different to the atmel chip in that it now has many MB of memory instead of just a few kB. Also, the sd card on a raspberry pi is persistent data storage, with more in common with the hard disk of a PC than with anything on the arduino.
Lastly, the clock speed of the raspberry pi, 700 to 1300 MHz at the time of writing, is more suited to the tasks of a PC or smartphone at 3 Watts, whilst the arduino, running a smaller cpu at typically 16 MHz, is using much less power.