Worth upgrading from 450d to 600d or 650d?

Hello everyone

I have had a Canon 450d for some years, shoot in raw, and edit in Lightroom.

I am considering upgrading my 450d for something with better picture quality, in particular at higher ISO levels.  In particular, I was impressed by a friend's night portraits, taken without flash, which were much sharper and less noisy than mine (he was using a 5d ii, I think).  He tells me that any recent camera will not only have much higher ISOs, but will produce much higher quality pictures at each ISO level, compared with my 450d.

I always think picture quality is largely a matter of lens, not body, but ISO performance does seem signifcant too, hence my thinking I might upgrade, probably to either the 600d or 650d.

So my questions:

1.  Is the raw picture quality at higher ISO levels in the 600d/650d far better than the 450d?  Is that worth upgrading for?

2.  What about the other determinants of picture quality, e.g. focusing, sharpness, etc.  Will these be appreciably better in the 600d/650d compared with the 450d?

3.  Regarding both of the above questions, if it IS worth upgrading, then to what?  In particular, which of the 600d and 650d is better regarding picture quality, esp at higher ISOs?

The DPR review camera comparison widget seems to suggest the raw picture quality is slightly better in the 650d, but noise handling at higher ISOs is slightly better in the 600d.  Is that right?  Is it a significant difference?  If so, that's frustrating, since I like the idea of the 650's touch screen -- but weight picture quality, if the difference is appreciable, more than ergonomics.

4.  Finally, other options include the 60d and 7d, but these are bigger and heavier, and I guess I think that, if I am going to go bigger and heavier, which I might one day, I might as well go full frame, perhaps for the 6d.  Sensible?  Or not?

Anyway, thank you so much in advance for any advice you can give.

Really, my main issue is whether to upgrade at all, on the basis of picture quality, esp ISO performance, and if so which of the 600d and 650d to upgrade to.

Thanks

David