It will not be like having one of those high end cameras. But, the body has one of the smaller roles in image quality: better lenses on an older body will blow away a crappy lens on a newer body. IMO the lens is far more important than the body is and all of the modern digital camera bodies are capable of fantastic results.

Look in my gallery, I am not saying I am that great of a photographer, just demonstrating the IQ you can expect from good glass on older bodies. Most of my gallery is taken with a G2 or 10D. Some with an S2, Some with a Rebel XT, and some with a 5D. All of which are not the latest greatest. The glass is old too, but some of the best available from Canon.

Remember the 30D was a high-end camera(well mid end)just two years ago. It was only replaced with the 40D about a year ago. It is still a very capable camera and will render excellent photographs.

A newer body, such as the Xsi, or 40D has some advantages:

-More megapixels (before I get hanged, I will qualify this statement: More MP is generally regarded as a "better" thing. It isn't necessarily a better thing, first more photo sites (pixels) crammed onto a sensor of a given size requires those photo sites to get smaller to fit, this increases noise, and decreases the amount of detail each photo site can hold.)

-Digic III processor. This is a fast processor than the Digic II processor. One of it's jobs is to help combat the noise created by the more saturated sensor.

-Slightly higher fps. Faster shooting, larger buffer. This can be helpful, but in my experience, except in certain situations, this just means burning up more space on your card.

There are also other differences, each may or may not be useful to you. DPreview has very in depth reviews of all of the cameras I mentioned. Read up on the features and decide which will be useful to you.