I'm going to disagree slightly with McBeck's explanation of Canon's lens range.
Canon has a very wide range of lenses, with plenty in-between the budget lenses and the L-series lenses. In my experience, there are usually three choices with Canon lenses; budget, medium, and L-series.
The budget lenses are very cheap, but still produce good quality images if you understand the limitations of the lens. Usually they will have small maximum apertures (f/4 to f/5.6) and slower focusing (the old arc-form drive or micro-motor focusing). Examples: EF 50mm f/1.8 II, EF-S 55-250mm f/3.5-5.6 IS.
The next step-up usually gives faster focusing (using either of Canon's USM focus motors), but keeps the small apertures. Historically it also added things like IS to the lens, but that is now found on budget lenses too. Examples: EF 50mm f/1.4 USM, EF 70-300mm f/3.5-5.6 IS USM.
The top of the range are the L-series, which give optimum optical quality, large apertures (f/1.2 for primes, constant f/2.8 for zooms) and very rugged build quality. You also get the full ring-USM focusing in most (all?) cases. Examples: EF 50mm f/1.4L USM, EF 70-200 f/2.8L IS USM.
The kit-lens which comes with the 450D is fine, as long as you accept that it's slow to focus and won't be very good for low-light photography. If you primarily shoot landscapes, it's fine. It's only when you need to do something your current lens doesn't do that you should upgrade.
I've had a 400D for a couple of years now, and I'm reasonably happy with it. The new 7D would be ideal for me, but I can't justify the cost yet.
The buffer will fill up if the camera is capturing data faster than it can be written to the memory card. At that point, the camera will stop taking photos until enough of the buffer has been dumped to the card that it can fit in one more photo. On my 400D in RAW with a Sandisk Extreme III CF card, I can shoot 9 images in a burst before the buffer fills up. At that point, it starts taking shots about once every 2 seconds. It's a good 20 seconds or so before the buffer is completely empty again.
The f/# number you see on lens descriptions is the maximum aperture of the lens. It's the size of the hole through which the light travels to the sensor. A larger hole means more light, and a shallower depth of field. Smaller f/ numbers are larger apertures. If you see a lens specified like EF-S 18-55mm f/3.5-5.6 IS, that means that it's f/3.5 at 18mm which changes to f/5.6 at 55mm. If you zoom to, say, 30mm, you'll probably get f/4. You can always choose a smaller aperture (f/8, f/11, etc...) if you want, but those will be the largest you can get with that lens.
Hopefully that explains some of it
