That is correct, only has to cover the 18-55 range. Right now, I am being pushed in the Sony direction but from what I read, the quality at higher exposure levels (ISO Levels) tends to be a bit noisy when compared to the somewhat more expensive canon. We were looking at the Sony A200 and the Canon 40d.
A lot of fuss is made about high ISO differences, but most photos are taken in low ISO, and the differences between the brands at high ISO are actually very small and not easily noticed on prints at most normal sizes. Any difference in quality between the lenses on the cameras will be greater.
The counter argument tends to be that Canon photos look 'plastic' because of the in-camera noise reduction which smoothes things out. (And whatever claims are made, ALL digital cameras do have in-camera NR)
You always need to think whether any differences important to pixel peepers will be differences you will actually notice in your own real life use.
High ISO photos on Sony are generally best taken in RAW and not PPd with ACR, but again whether you will notice the difference is pretty moot.
If you are worried about high ISO, download copies of photos you like taken by the camera in question at high ISO and have them printed at relevant sizes (or just check them out on your monitor - worth remembering that not all monitors are really worth trying to calibrate, so worth thinking about monitors too - potentially $£$£). Remember a lot of stuff on the web is either too small in resolution, or done at 100% making it too big.
You have to be really cynical when reading stuff on the web about cameras, whether they are reviews or comments. Most, if not all, are done by people with a commitment to one system and much less experience of others.
If you are interested in Sony, it is worth visiting photoclubalpha
. The photog who runs the site has extensive experience of Sony and Minolta - but also of other systems - and one of the most frequent posters seems to think of nothing but extremely high ISOs.