I would probably go with ATmega328P but you'll be fine with ATmega328 if that's what you end up with as long as you're not needing super low power operation to make batteries last for a long time.
Infraviolet:
That one from farnell looks a bit odd though, the data sheet seems normal enough but the image shows it to be rather wider than usual PDIP chips, and there isn't anything on the webpage itself which explicitly mentions pin spacing or width of the device. The datasheet talks of pin spacing and seems to say it is as usual, but for all I know farnell might have taken a standard datasheet for the atmega328p and accidentally put it on the page for a 328p in a wider format.
I think they just messed up and posted the wrong picture or maybe they just put a generic IC picture on their products so they don't need to have a specific pic for every single one. The ATmega328P is only available in one DIP package. Farnell is reputable so I'm sure they'll send you the part you order. Note that is only available for backorder. That is the ATmega328P-PN, which has a higher temperature rating than the more common ATmega328P-PU. You might find the -PU to be in stock and cheaper. Either will work interchangeably other than the temperature rating.