40D does have a metal chasis. The XXD is the line that's intended to be a step up from the Rebels. Whatever you do, don't bring MP into the equation. I have a 300D that I can very well still print off 30"x45" enlargements because I use a quality printer and edit to their specifications. It's 6mp. It's not my main camera, but I still threaten to bust it out just to shoot with the guys that have the really expensive gear.
Seriously go play with them. I had an XTI as well and after moving to the 30D, I don't really like shooting with the rebels. The XXD series has a top mount lcd and controls which allow you to change settings like ISO without going into the camera menu. You also have a scroll wheel which lets you change the aperture without having to hold down an additional button and use the shutter wheel to change it, like on the rebel line. They also have slightly faster AF, iirc. I don't know if they changed this on the XSI, but on the previous Rebels, you can only change ISO in full stop increments, so you're limited to 100, 200, 400, 800, 1600, where as on the XXD series you can change it in 1/3 steps, which gives you a higher degree of control.
That's what the next step up in Canon's line up is about though, better control. And IQ will be the same. They both use the DIGIC III processor. Also, the XSI is the first camera in Canon's line up that only uses SD cards and doesn't accept CF, which every other current camera (unless the XS is out at the moment) takes CF cards.
It's up to you, but I like to reccomend the better body. You can always purchase the 40D without a lens and buy seperately. That or look at a 30D. They're going for about $700 new, iirc. That's a good deal.