The part number format is ATMEGA328x-yy. The X identifier can be: not there, 1 letter, or 2 letters. Those with 2 letters such as U2, M1, or C1 are a completely different chip than the ones used in standard Arduinos. The only suffixes of concern are the letter 'P' or nothing at all. The 'P' stands for picoPower which is ATmel's line of very lower power chips. There is usually very little price difference between the regular Atmega and the picoPower version, so for most projects, the picoPower version would be the best choice, as that gives the option for extremely low-power battery-powered applications.
For the last two digit suffix, the first digit stands for the package type. The choices are:
- A - TQFP
- M - VQFN
- P - PDIP
- N - industrial B, -40 to 105 degrees C
- U - industrial A, -40 to 85 degrees C
Now you know what chip to buy.
For more information on these distinctions, see the Atmel page for this series chip.