Lets cover this in a little detail..
The kit lens I presume that you are talking about will be the 18-55 mm that the majority of people already had just to get them started. Its available bundled with the 300D/350D/20D and as such many, many people now own a copy of this lens.
As a beginners lens it is excellent, it covers a range that I would assume you would be shooting at with your interest in landscape and to replace that lens with something that has a similar range but better quality will cost you a lot more..probably as much as £600. I have seen some excellent results from the 18-55 but I have also seen some shocking ones as well. The key to this is make sure you get a good example of this lens and also make sure that you learn your equipment to get the best from it.
I wouldnt discount getting the kit lens just to start you off.
The next choice I would recommend is the Tamron 17-35 F2.8 XR DI IF, lots of letters there but underneath it all its a cracking wide angle lens thats both sharp, fast, small and light. The results that are possible from this lens are far beyond its cost would suggest. It can be bought in the UK for around £360 last time I checked.
The next step up will have to be the Canon 17-40L, this widely accepted to be a very high quality lens and many people are fortunate enough to own one. The quality of this lens is unquestionable but as it comes from Canons premium/pro range it carries the price tag to match, if you shop around and are lucky you may be able to find a new one for around £550. Secondhand examples are rare. The build quality is excellent and its a weather sealed lens as well, when used on the pro range of Canon cameras it allows both to be weather resistant.
The next stage would be the 17-85IS that CT has covered above, it has a longer range than the two I have mentioned so far, has Image stabilization but is not as sharp as the 17-40L and the cost will be similar. You will have to weight up the benefits and pitfalls of that lens compared to the others and how it fits into your idea of what you will use.
The last lens I would like to mention (for now) is the brand new Canon EF24-105f4 L USM IS, this is not available just yet and was only announced this week. The benefit of this lens is that it has a wider range and probably most useful to you than all the others mentioned here, it has IS and benefits from being an L lens as well. Basically its the best that Canon can do at this time, it has all their latest technology, has the best optics that Canon can make and will undoubtedly be made well as a lens. There are no examples of pictures that have been taken with this lens yet as its so new, in fact people are still guessing at the price, but it will be worth keeping an eye open for reviews and once the price has been set you will be able to judge if it should stay on your short list or not.
There is a lot of information there to swallow, if you have any more questions just shout.
