So I am going to purchase a 50 watt jcm 800 2203 in 2 or 3 weeks depending on when i get the money, the guy is holding it for me. I am also going to get a cabinet from somewhere, not sure where yet possibly Avatar or Lopoline or I will look used.

My question is would two Celestion Greenbacks be able to handle 50 watts, or would it be safer to go with speakers that can handle more than 25 watts each.

If there is a thread with this information please direct me to it as I am unaware of the existence of such a thread at the moment.

MIA Fender Stratocaster
Jackson Kelly KE3
Ibanez Universe
Marshall JCM 800
Orange Jim Root Terror
Mesa 2x12
Ovation Custom Balladeer
pedals and such in profile
Well for a start, the Greenback is only 20W.

Since wattage is a complex thing and manufacturers often measure it in difffernt ways, it's usually best to err on the side of caution. If you crank your amp regularly, it probably will shorten the life of the greenbacks, though it'd still be years before it did any real damage.

If you've got any doubts you could always try a V30 and a Greenback in the same cab - a great 'best of both worlds' combination, especially with Marshalls. A little of that Greenback crunch will get drowned out by the mid-heavy V30, but it's a decent pairing for classic/hard rock.