Why are people down-voting this? Does it violate some guideline? If you don't think it's a good question that's fine, but I think this is a legitimate question that many people have.
This is the kind of thing that review sites like dpreview pore over in excruciating detail. Or if you just want a summary, try snapsort: http://snapsort.com/compare/Canon_EOS_Digital_Rebel_XS-vs-Canon_EOS_Digital_Rebel_XSi
We can point you to those reviews or comparisons (as the top answer you've gotten so far does), but Google can probably do an even better job. If photo-SE starts getting top hits for that question when going to the reviews directly would be better for searchers, we're making the internet worse, not more awesome.
Addendum: Almost anyone who is going to answer this sort of question will probably start by doing a search or looking on one of the big camera review and comparison sites. (Even someone who is familiar with both models will probably have to look up specs.) That's a sign that the question isn't adding much for anyone.
Also note that the rough guidelines have been clarified since, as the tooltip text on the downvote arrow states now:
This question does not show any research effort; it is unclear or not useful (click again to undo)
→ More related discussion in the latest podcast/blog entry on blog.stackoverflow: SE Podcast #04
Unfortunately, your question fills the "requirements" for a downvote. Furthermore, without any context, no one else can edit it and still be sure that the edit will respect your (the original author's) intentions.
In addition, what kept me from answering it was:
It's too general. Beyond comparing the technical details — which are rather trivial to find — what are you really asking for? It would've felt unnecessary just to copy-paste the details from Canon's website (for example).
It's too specific. Should I provide more general answer between the different models in Canon's lineups — e.g. what are the general differences between 0000 D models and 000 D models? This would provide a more timeless answer; but would it help you enough? Especially since three new models have been released in the 000 D line after 450D, and you specifically asked 1000D vs. 450D.
So, showing no research effort and being too general and too specific at the same time, I felt it needed a downvote*.
Still, I believe the problem is real. I myself compared 1000D and 450D almost to exhaustion (but didn't come to any real conclusion as I found an almost-new second hand 450D at a bargain price before it) approx. two years ago. And actually, technical details didn't help much as 1000D is essentially a downgraded version of 450D. Votes aren't permanent, though, so if the question gets improved, I, for one, could vote it up. (Although this might be improbable as an answer is already marked as accepted.)
*) NB: actually I downvoted at the same time I wrote this answer as I wanted to give some more feedback beyond the comment field and this meta-question gave the opportunity
Addendum prior to Post Your Answer
OK, a great answer could've included both specific and general points — but a great answer wouldn't make the actual question better.