Highest usable ISO between 5D and 5D Mk 2 & grid

I shoot a lot of street photography, often at night or late evening, so I'm a bit of a fast lens/high ISO freak. I'm thinking of moving from my 450D (Xsi) to either the 5D or the 5D Mk 2, right now my preference is to the newer 5 but I need to know if I'll be getting significant increase in higher ISO performance over the old 5.

Also the new 5 has a newer matte screen for manual focusing; has any one used it & your thoughts on it would be appreciated.

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An excellent lens lasts a lifetime, an excellent DSLR, not so long.