APS-C size image sensor? (noob with new 400D on the way)

This is my first post, I thank everyone in advance for their replies.
My new stuff will be here next week. I ordered a 400D body, 580EX flash, Sigma 17-70mm f/2.8-4.5 DC MACRO lens, Sigma 70-200mm f/2.8 EX DG APO HSM telephoto zoom w/MACRO AF lens and a 12 GB memory card.
My questions are: Will the 17-70 lens work with this body? The Sigma website says: "It can be used with wide variety of subjects and it is an ideal large aperture standard zoom lens for digital SLR cameras featuring APS-C size image sensor." What does that mean, APS-C size sensor?
Another question: After digging through this forum, I think I got too large of a memory card. It seems most have several smaller cards of 2 or 4 GB's. The card seemed like a deal for $199. Seems everyone is concerned about losing their pictures from a card failure.
Last question: I plan on shooting some action shots like BMX racing and ice hockey. Also I plan to shoot outdoor shots (landscape and wildlife). Any suggestions on what else I could use would be appreciated.

Yes it will work. APS-C sensors are smaller than a 35mm film frame. For comparison to a 35mm frame they give you a 1.6 narrower field of view. So the 17mm will be similar to a 27mm field of view on a film camera. Its called a crop factor. The 17-70 is actually designed for APS-C lenses only, if you mount it to a film camera it will vignette badly in the corners at the wide end of the lens. The card is a personal choice, just go with a name brand card.

oldbmxer66 wrote in post #3885288
I ordered a 400D body, 580EX flash, Sigma 17-70mm f/2.8-4.5 DC MACRO lens, Sigma 70-200mm f/2.8 EX DG APO HSM telephoto zoom w/MACRO AF lens and a 12 GB memory card.

Nice selection!
What else might you want? How about a 10-22 for wide landscape work, and a decent tripod?

oldbmxer66 wrote in post #3885288
What does that mean, APS-C size sensor?

There are 4 sensor sizes being used in Digital SLRs. The APS-C size (around 22x15 mm but varies a bit between brands) is used in all but a few models. This size is often referred to as a 1.6x (Canon), 1.5x (Nikon), 1.7x (Sigma), or something in that vicinity.

The other sensor sizes found in DSLRs are:
4/3 (18x13 mm, 2x): Olympus and Panasonic
APS-H (29x19 mm, 1.3x): Canon 1D, 1D mk II, 1D mk II N, 1D mk III
"Full Frame" (36x24mm, 1x): Canon 5D, 1Ds, 1Ds mk II, 1Ds mk III, and Nikon D3

The APS-H size is also used by the Leica M8, but that's a digital rangefinder camera, not a DSLR. Some old Kodak DSLR models have full-frame sensors.

http://www.cnet.com.au …ide/sensor_comp​arison.jpg

http://images.google.c​om ….microsoft:en-us%26sa%3DN

Yeah, 12GB is a little excessive. It would also be cheaper to get three 4GB cards. You can get a 4GB CF for as low as $40. Check out Newegg.com; they have great deals and are very reputable.

I have two 4GB and one 2GB and the only time I ever filled them up was at an airshow using two bodies.

You'd get well over a thousand images using 12GB. Can you see yourself ever taking that many without having the opportunity to download them to your computer? If not, I'd skip it. Besides, it is not wise to have all your eggs in one basket so to speak.

I'd spend the money you save on some more hard drive space for your computer. You'll need it if you're dumping several GB of photos on it per week. And besides, hard drives are dirt cheap compared to flash.