Honestly, either the 1000D or the 550D will be a big, big step up from a PowerShot. You're correct in that camera bodies become outdated relatively quickly - but that's outdated only in the sense that a new one with more/different/'better' features comes out. If you're happy with the pictures from your camera, there's no need to upgrade to the newest body. Lenses are different - a good lens will last through several bodies, for those who tend to upgrade.


Either body will allow you to learn the basics of exposure (aperture, shutter speed, and ISO settings).


I'd definitely encourage you to get both a zoom and a fast prime lens (the 50mm f/1.4 is very nice, and will give you a taste of better lenses; however, the 'nifty-fifty' 50mm f/1.8 is a good value, too, and cheap).


How sure are you that photography is going to be your thing? If you just want to 'try it out' with a dSLR, then the 1000D + 18-55mm kit lens + 50mm f/1.8 is the least expensive option. You'll still be able to get some great pictures and learn a lot in the process. If you decide to make it a dedicated hobby or more, you can increase your investment later.


Just as a side note if you go with option 1 above, and sorry for adding another variable, but instead of the 50mm f/1.4, you might consider the 85mm f/1.8. It's also a fast prime with great image quality, and that way you'd also have a longer lens in than your 18-55mm for around the same cost as the 50mm f/1.4.