The L lenses you're looking at, and the 24-70/2.8L II and 24-105/4L IS USM, are designed for full frame. The 17-40L is not meant to be a walkaround zoom on a crop (though you can certainly use it as one), it's designed to be an ultrawide zoom on full-frame.

You may want to consider the crop analogs to the 24-something L lenses, the EF‑S 17-55/2.8 and the EF‑S 15-85 f/3.5-5.6 IS USM. If you're willing to compromise a little farther on image quality for a wider zoom range, there's also the EF‑S 18-135 f/3.5-5.6 STM.

24mm, on a crop body like the 450D, is not particularly wide, and you might find it lacking for landscape use. Look at the landscape shots you've taken with your kit lens and see how often you're using 18-24 before making the decision to go with a 24-105 or a 24-70. And if you do go with a 24-something L, then you might also want to consider adding an ultrawide lens, like the EF-S 10-22 or Tokina 11-16/2.8 to fill in the wide end of the range on crop.

Ls are great, but they're the most expensive, heaviest, and largest of the Canon lenses. And they aren't that much sharper than the gold-ringed USM lenses. Ls have better contrast, build, and usability features, but the image quality gap is narrower than some people think. They may cost three times more, but they're not three times sharper. Buyer's remorse can hit particularly hard with a first L simply because of the price tag and inflated expectation. For example, the 24-105 is the 5D/6D kit lens. It's often considered to be a "compromise" lens--medium fast max. aperture, weak at the wide end (for a $1000 L zoom) when it comes to chromatic aberration and distortion. No lens is perfect, not even an L.