Stetson
One major difference between the Rebel and the xxD series is the buffers. The speed with which you can take multiple pictures with the 20D is mindboggling. The Rebel, fairly slow.
In the context of the conversation for a new DSLR user that seems a fairly moot point. While the 40D has a faster burst rate than the XTi it is also a lot faster than a 30d. Technology is evolving at a staggering pace. Burst rate really says very little about over all IQ which is far more likely to be the main focus for most. I'd be a lot more interested in continuous shooting rates Vs burst rates as this is going to have a far greater impact if action, bird in flight or sports photography are your primary interest.
For any one starting out cost is likely a big issue and IMO glass is far more relevant than up-graded bodies. The amount of residual expense in this hobby is staggering. Software, tri-pods, BH, Bags, cleaning suplies, cards, etc.
I can not think of a time I have ever lost an image on the XTi due to burst rate or even continuous shooting rate for that matter and 99% of the time I shoot Jpeg and RAW. The sheer number of people using poor quality cards probably has more real world effect on this than actual body performance.
There are, at least for me, several upgrades that make the 40D a worthy addition but honestly burst rate as well as some of the other marketing stuff such as live view and partial weather sealing are pretty low on my list.
The major improvements I see;
The viewfinder depending on the size of your wallet could be worth the price of admission alone. Improved AF X point sensors at f5.6 or faster plus improved center sensor with f2.8 or faster glass and multiple exposure MLU.
