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Canon’s naming convention for its dSLR bodies is quite simple. Here’s how it goes:

Single digit numbers refer to the high end EOS bodies:

- 1Ds Mk III, 1D Mk IV (professional dSLRs with full weather sealing. The “s” suffix refers to full frame, those without are the APS-H (1.3x crop factor) bodies.
- 5D, 5D Mk II (high end full frame bodies)
- 7D (high end APS-C format body)

Double digit numbers refer to midrange or prosumer bodies with partial weather sealing and magnesium alloy exterior:

- 30D, 40D, 50D. (all are APS-C crop frame bodies)

Triple digit numbers are Canon’s consumer level bodies with polycarbonate exterior and stainless steel frame :

- 400D, 450D, 500D, 550D (all are APS-C crop frame bodies)

Only one 4-digit EOS model exists serving as Canon’s entry level model:

- EOS 1000D (APS-C format)

Features wise, the 50D (overdue for a newer model now) has an updated DiGIC 4 processor, 15MP CMOS sensor, a high resolution 920K VGA rear LCD, lens vignetting correction and a micro AF lens calibration adjustment up to 20 lenses.

The 50D won’t win any awards in the high ISO department, it’s said to be prone to noise pattern banding especially if you push the sensitivities higher. Image wise, unless you need more detail (15MP vs 10MP in the 40D), you won’t miss much in your present 40D. Take a look at Digital Camera Resource’s high ISO comparison between the 40D and 50D and you’ll see that the 50D has a loss in dynamic range and color saturation at ISO 3200.

You didn’t mention what kind of photography that you do, but if you need HD video and a low noise sensor at high ISOs and a lighter body, the 550D won’t disappoint you. IMO, the 50D is an incremental upgrade over the 40D (not quite a replacement) compared to the 7D, which has a big jump in features and image quality.

I don’t think the 5D Mk II or 1D series will drop in price to affordable levels as they are high specification cameras with an equally high cost of production. Being expensive cameras to begin with they don’t sell by the truckloads like the popular Rebel series, consumer dSLRs.

You might want to wait till the 4th quarter of this year, it’s said that Canon will come out with a model that bridges the gap between the 7D and the 50D. Many folks predict that Canon will name it the 60D, while others opine that the 7D was supposed to be the 60D but they decided to rename it as the 7D.

Another suggestion is to look for a gently used, pre-owned 1Ds Mk III, 7D or a 5D Mk II. And for your final question, yes – any EF and EF-S lens can be used with a Rebel dSLR. I’ve tried an EF 70-200mm f/2.8L IS USM on my 450D and it’s very hard to grip the camera due to the 450D’s small hand grip. Can’t say I liked the feel as the whole gear felt front heavy. I had to balance it with my BG-E5 battery grip for better feel and handling.

IMO, a heavier body such as your 40D would be more suitable for a 1.4kg lens like your L-series EF 70-200mm.

Hope this helps.

Also you can use the Canon 70-200mm 2.8 IS II on the 550D, yes?

100% agree.

By using the same perspective, most of the consumer will love to have this video capability for their home video or vacation even with some shaky video here and there.

The fact that it can take both decent still pictures and video itself already sold many consumer to buy this camera (it is true that a handycam can take still pictures too, but I believe the quality is still far from the quality and flexibility of dslr camera).

Yes, it’s interesting to note that adventurous film makers are trying out the video capabilities of the EOS 5D Mk II, 7D and the T2i. That should be great for amateur film makers on a tight budget. I read about a film project involving the 5D Mk II, but the videographers weren’t using EF lenses for the movie – they mated the EOS body to Panavision lenses.

I don’t suppose any 550D user would buy one with a Steadicam rig in mind for their vacation or home videos. Neither would film makers hand holding such dSLRs without adapting them to specialized video rigs. There are still limitations with dSLRs as primary video filming equipment. I’m of the opinion that movies shot on 5D Mk IIs are purely for experimental purposes and Canon also benefits from such media exposure to sell more of their dSLR products.

You don’t see major movie producers using Canon dSLRs for their movies. It’s usually Panavision film cameras that they choose and TV series producers that use digital format would use professional digital cinema equipment like Red One cameras. Students in the field of cinematic arts would more likely benefit from using affordable gear like the 5D Mk II for their short film projects.

A 550D used for video by embedded photojournalists in hot spots like Iraq has less chances of surviving brutal use in harsh conditions. Rebel dSLRs are consumer grade cameras are not made to withstand hard outdoor use.

Thank you for your comment about my blog . I agree with you that it will difficult to keep the camera steady when shooting a video. On the other hand the video capability of DSLR camera that gives a filmic looking video attracts many filmmaker to use it in their project. They have the right tools to make the camera steady and gives them a superb result.

Here is one of example of the clip from “The Bridge” which shot by the T2i/550D camera

http://www.eos550d.com/2010/02/the-bridge-a-positive-confirmation/