Thanks for the replies!
I decided to go with the HD650s in the end as I figured that if I upgraded the rest of my kit I wouldn't need to upgrade them! I have tried them with my iPod and you were right, they do sound a bit lacklustre compared to when plugged in elsewhere. I'll just stick to using the HD477s I have with the iPod untill they are totally knackered I think.
In the meantime, I am really in love with the sound these HD650s make, lord knows how they will sound when I can afford a headphone amp!