Better still, just get the serious one straight away!
But really, good lenses retain their re-sell value very well. So after using it for awhile and you prefer something different, just re-sell it again on eBay. Consider the price difference is the price for "renting" the lens.
But if photography isn't a big hobby of yours and just wanna take some nicer pictures than P&S, then I think the kit lens is enough.
But still I would rather have 18-70mm kit lens than 18-55mm kit lens. The quality is different, both build and image quality.
I haven't got the lens anymore, but as far as I remember, the difference in build are : 18-55mm is full plastic even the mount is plastic, the MF focusing ring is that tiny little ring right in front of the lens (once you've touched it, you'd switch to AF straight away I guarantee), there is no distance number on the body unlike 18-70mm (although I don't really utilise it much but it's cool to have numbers. you can't have enough numbers
), and I think it's lighter than 18-70mm. Anyway, when you hold both you will notice. But they both still feel crappy compared to old metal AI lenses.
But to be honest, the 18-55mm can take pretty good pictures, I just can't stand the build.
Have you read some reviews of the lens? Try Thom Hogan's reviews here:
18-70mm
18-55mm
Might help you decide.
PS: Sometime Ken Rockwell might sound a bit over enthusiastic, so make sure you read several independent reviews.