Here is a great website that does in depth reviews of almost every camera that is out there as well as having a side-by-side comparison and lots of sample photos. They have G10, all the above mentioned DSLRs and the panasonic mentioned as well as Nikon, Sony, Pentax, Olympus, etc.. This website has a TON of information.
http://www.dpreview.com/
The main drawbacks I found to the XS vs the XSi.
-No spot metering
-Less auto focus points
-Less megapixels
-smaller LCD
-plastic grip
-no IR reciever
-no viewfinder sensor
-1.5 FPS in RAW mode
I picked the XS over the XSi because it was about $200 cheaper for the XS and none of the drawbacks seemed to make it worth it to spend the extra money. No spot metering is a bummer, even though i don't know what it is, but the other stuff doesn't make a difference to me. I was able to try out the XSi before buying it and the grip definitely felt alot better. It was bigger and filled out my hands better, the grip on the XS was alot smaller and feels like it would be a better fit for women or children.
I didn't go for an XTi for a few reasons. No live view, no auto ISO, compact flash was a negative for me as I already had a 16gb SD card, smaller battery and no image stabilization lens.
Please don't feel like I'm trying to sell you a camera. I'm just sharing my own findings with you. I implore you to explore that website. Use the side by side comparison on all the camera models in your price range, SLR or not and look at the sample photos, read the reviews.
Also a word of advice when buying things used, if you decide to go that route, you will receive no warranty. You may get a better deal, but at the same time you may get a worse one. These cameras can get ruined pretty easily if the previous owner didn't take proper care of it. At the same time though, if you're patient and look hard, you can get stuff that is basically brand new for pennies on the dollar.
Good luck!