I would guess the ATMEGA644P-20PU, but I'm not sure, so I thought I'd ask...
I think that's right - the first one would also be OK likely, but the PA (IIRC) means "automotive temperatures" or something like that (able to survive the extremes of that environment). The PU (once again, IIRC) means "DIP" package, while PQ means the QFP-style package (SMT).
I think.
:-?
[edit]
Hmm - now that I look at Digikey, what I said above likely isn't correct...though I still think the one you are talking about is likely the one needed...[/edit]