Is the Canon 17-40mm L lens good for architecture and real estate photography - Absolutely. Keep in mind that especially at 17mm you will need to remove the barrel distortion in post processing. If you are especially worried about this, and want to take the extra time and attention that it requires, you might be interested in tilt shift lenses or perspective control lenses. You can shift and fix converging lines with these, which can be a huge advantage with architecture. If you want to shoot indoors without a flash(you don't for almost all real estate photography) you can look at the more expensive Canon EF 16-35mm f/2.8 L II USM lens, but I wouldn't.

The 5D MkII is also an excellent choice for real estate or architectural photography. One of the main reasons that this is true is that the full frame sensor will give you more options when it comes to wider lenses. For example a 17mm tilt shift Canon lens on your 550D will have the field of view of a 27mm lens on the 35mm format. On the 5D you can actually use that full 17mm and get a great indoor shot. This is a huge advantage.

I think it is important to understand why you feel that you need to upgrade. The lenses that you point out aren't ones that you can only find on full frame cameras. The 17-40 is roughly equivalent to the Canon 10-22mm lens that is already produced for crop sensor cameras. If you are upgrading for the ability to use a lens like the 17mm tilt shift Canon lens and have the field of view of 17mm as outlined above, that is a great reason to upgrade. Beyond that, I think it would be of much more value for you to invest your money into additional off camera flashes, preferably the Canon 580EX II with a few pocketwizards as an option.

Another item I feel is worth noting, is what type of photography are you actually pursuing? Are you shooting in volume and only looking to increase your throughput? After your comment on "speed of processing" I think this may be your area. If you truly are trying to capture the high end segment, then you really need to invest in a wider range of lighting to get great indoor shots, that, along with time and attention to detail, is where the high end market lies. I don't think your current body and lenses are limiting you in achieving that.

I leave you with an article that compares "Architectural" photography and "Real Estate" photography by my friend Mark Teskey. He is a local photographer and does excellent professional architectural work. He has a very interesting perspective on the trade that may influence your desire to upgrade your camera body, or look at other options such as workflow enhancements or lighting upgrades.

Summary

• The 5D MkII & 17-40mm pair excellently for real estate photography.
• The body/lens that you currently have also pair excellently for real estate photography.
• You may benefit more from investing in lighting equipment then a new body.
• Consider if you are photographing to "shoot and burn" or if you are photographing to create a true work of art to be admired. You may be investing in the wrong place.