I think the Xti is a great camera. The main draw to my 40D is that the camera's functions are much more easily accessed. It is also all around built solid. I think it would take a near identical picture as an Xti though.
With the lenses, you are going to want a 100mm f/2.8 for sure. This will be your coral lens and can be had for around $500. There are way to many additional lenses to list, but if you know you want a 5DmkII in the future, here are a few I would recommend checking out:
Zooms
*16-35 f/2.8 or 17-40 f/4
*24-70 f/2.8 [I own] or 24-104 f/4
*70-200 (any of the 4 versions are excellent) [I own the f/2.8 IS]
Then you have the prime route. These lenses don't zoom in and out but can generally be had at a lower price (per lens) while giving higher quality.
14mm f/2.8
24mm f/1.4
35mm f/1.4
50mm f/1.4 [I own] (the 50mm f/1.8 is a good starter lens)
85mm f/1.2 (for better picture, not to mention it is the sexiest lens in the world IMO) or 85mm f/1.8 (for faster auto focus) [will own]
135mm f/2 [will own]
200mm f/2 IS
Of course you can mix and match, and there are other options out there, but you get the idea. All of the lenses above will not only work with a 5DmkII, they will work amazingly well.
The 5DmkII is a heck of a camera and it hungers for high quality lenses to match because of its full frame sensor. In the Xti or 40D, the edge of the image is cut out and discarded by the camera. It is never seen by the photographer. This is called a "crop factor" and is the source of confusion for many. The 5D's lure is that this "crop factor" is eliminated, imitating 35mm film. Having a full frame camera gives several benefits. For one the angle of view for a given focal length is substantially widened. For two the extra space gives the megapixels in the sensor some leg room. Long story short they function better which results in a higher quality image with less noise from ISO speeds.
The drawback to full frame sensor is the result of its appeal. The sides of the images are not cut out. Camera companies sell a wide range of products to a wide range of photographers. Canon for instance has point and shoot cameras which can be had for between $80 and $400. Depending on how much money you are willing to spend and/or how much quality you want, they try to cover the bases. DSLRs are no different. The main reason the 5DmkII so expensive by the way is the sensor. The material the sensor is made of comes in a silicon "wafer" It looks something like a shiny pancake. A VERY EXPENSIVE shiny pancake. IHOP doesn't even serve them. The larger the sensor in your camera, the larger space must me cut out of the silicone wafer. If there are any defects in the wafer piece, it must be thrown away and chucked as a loss. I bet you would be ****ed if you paid $3000 for a camera and it had red and blue dots in every picture straight from the box right? Well the greater size of cut taken from the wafer, the greater chance of getting a defected portion. This is one reason why larger sensors are EXPONENTIALLY more expensive than smaller sensors. A sensor twice the size may cost 4 times the amount of money to produce.
The cheaper cameras (1000D, Xsi, 50D) have a 1.6x crop factor (because the sensor is smaller). The sides of the images are cut off. Common logic states that the buyers of cheaper cameras will want to buy the cheaper lenses. Since .4 of what the camera sees through the lens is being cut out and thrown away on the cheaper cameras anyway, why worry about the quality of those corners if neglecting them will substantially reduce the cost? The low cost in turn makes the lenses even more appealing to their target audience.
Well the 5DmkII wasn't made for those lenses. If you pair it with a low quality lens, the corners and sides of your images may not look like you would hope. What is the good of a $3000 camera if the corners of your pictures look like crap? I just wanted to offer a little background into my expensive lens suggestions for the 5DmkII. Of course...I would suggest those lenses over any others for an Xti as well so...
ESPECIALLY for a 5D though.
