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Liz, if you already have a 450D, none of the cameras mentioned are going to offer much of a step up in low light performance and as JJJ points out many of them fall to pieces in high ambient temps.
If you're looking to do serious landscape night work without a tracking mount then you're really in to the realm of the 5DII ($$$) or similar and fast lenses to gain much on the 450D
My suggestion is make the most of the 450D you have and perhaps put your money in to fast lenses; a budget of $500 would just about buy a Tamron 90mm F2.8 and a 50mm f1.8; one stop faster in a lens is equivalent to doubling your ISO.
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