Enjoy your flight, I think you will be surprised at the size of the 739 cabin; if one didn't know better, once inside, you can easily confuse the 739 with a 757, its that long.
As for comfort, all of the comments above are correct, leg room is typical uncomfortable coach......not worse than the rest but tight, but your flight is only about 3 hours. I wouldn't waste money on an upgrade, KLM's European business class service is nothing special and it is possible, if you follow some of the suggestions above, to sit in a business class seat while flying coach - many tourists fly to ATH so its possible that business class will not be fully booked and some business seats (although behind the curtain) will be used for economy pax - maybe you will get lucky.
Its a shame that KLM did not invest in any IFE for the 738/739 aircraft.....while looking at the window is the best IFE in my opinion, the lack of audio and video is noticable on a flight of 3 hours or more, and somehow makes this brand-new aircraft seem old fashioned from a pax service point of view. Enjoy.