First of all, as has been pointed out before, you can make very good photos with any camera. Lens quality can become important for certain tasks, no doubt about it. But usually it's hyped: a well-composed photo will still be a great picture even if you have 2 % less brilliance or 5 % less sharpness on the corners.
Now, expensive lenses have other benefits: for instance, a larger aperture. There are three main reasons why this is a good thing: (i) In the times of film, you couldn't `change ISO' by the push of a button, you needed to change film. So in order to take pictures in low-light conditions. Nowadays, this is not really necessary anymore, you can simply crank up the ISO. My new D80's ISO range is 100-3200, my E-20's was 80-320, now that's a factor 10! People will say that at ISO3200 the noise is very strong and they're right. However, they forget a few things: noise can be part of a good picture (I for one like grainy film), or to quote David Carson: `Who says a good picture has to be sharp.' Also, the being able to take a picture (with noise) certainly beats not being able to make the shot.
Reason (ii) still persists and is actually why I bought a used 2.8 80-200 zoom: depth of field. Now this opens up a lot of new possibilities in taking pictures that cheaper lenses can't.
(iii) Lastly, more expensive lenses are much sturdier.
If you don't want to spend too much, but need these features, you can either buy used or (even better IMHO) go for either Tokina or Sigma. Tokina is IMHO superior to Sigma, the built quality is on par with Canon's L series or Nikon's pro lenses. The picture quality is excellent, usually either getting very close to the original manufacturer's lenses or in some cases, surpassing them according to various magazines (e. g. the 12-24 zoom is an excellent example). At the very least, they are cutting it close and from a price/performance point of view, they are usually the better choice by far. (Even some pros use Tokina and Sigma lenses.)
There are a few downsides to pro lenses, too. The biggest one is weight and size: my 80-200 Nikon zoom (bought it used) weighs more than 1300 g. While I don't mind the weight, you end up looking (and shooting) like a paparazzi. Changing lenses becomes cumbersome and it doesn't fit into my lens bag anymore. I will eventually replace it by a Tokina 50-135 zoom (not because of weight, though, but because the zoom range harmonizes much better with my D80). Another downside (this is true of older, used pro tele lenses) is that the AF speed might be slower (everything is heavier, you need more precision while focussing, etc.).
IMHO you should rather go out and try to focus on pictures. Don't think in terms of `this L lens will give me more sharpness' or test your equipment taking pictures of tiles and bricks. I took some nice pics with my father's 50-year old Zeiss Ikon range finder!
