Replying to Cropped sensors and effects on lenses?

I have the t2i which is has a cropped sensor of 1.6 from what I understand. The thing I don't understand however is really what effect that has on my lenses. If I buy a 5D my focal lengths will change I guess. And I'm really confused as to now why people talk about what focal lengths are best when really it all depends on your sensor right? My 50 from what I understand is like an 85 on the 5D? I don't know just if you have any knowledge of this just write some down. I'm trying to decide on my next prime to get for those dope video shots (I already have a 50mm) but then when I'm reading things I have to keep in mind that they may be talking about a different sensor crop or full frame? This shit just gets confusing.