I changed from a 50D to a 5D Mk2 a couple of years ago and find the 5D2 hugely better, particularly with noise. I struggled getting the 50D over ISO400 without a lot of noise visible, but with the 5D2 I can get very usable images at ISO1600.
Again, as has been said, we could do with knowing what sort of photography you do? Don't forget the 5D2 is a full-frame so make sure all your lenses are suitable and you'll lose the crop factor from the 50D.