Dont sell your 20D for the reason of "better IQ from the 450D" because this is all crap....
And each and every single Canon lens EF or EF-S will work fully well with the 20D!
Also, dont underestimate your 20D.. its a semi pro DSLR and it's sensor will definitely be more powerful than the consumer oriented XS/i!
I suggest save up and buy the 40D or the 40D Mk II or 60D when it comes up somewhere in the fall 2009...
As for features.. unless you do a lot of MF the Live View is useless!
Besides... You give me anything in the World taking only a pentaprism type ViewFinder and I'll deny... you wont even want to look in the XSi viewfinder after a while however small the difference.. trust me!