Just to give you an idea of raw images from a 500D, the following directory has three subs, 1 minute, 3 minute and 5 minute. They are direct RAW to JPEG conversions using the Canon software, with no other editing done.
WARNING: They are each around 10MB in size.
http://completelyserious.com/pics/as...2/raw_samples/
Images were done using an unmodified Canon 500D, Scope: ED100. No filters and a Western Sydney light polluted sky.
Just a few other notes:
From memory, the 1000D is missing spot metering, which may or may not be important to you.
The 450/500/550D also has more AF points than the 1000D, if this is important to you. (9 in the 450/500/550D vs 7 in the 1000D)
The 550D also has some nice video functions, though this may or may not be important. I'm not complaining about video performance on the 500D, though it's not something I have used much or really looked into.