Tokina AF 20-35mm f/3.5-4.5
One of the more useful indoor lenses, I have found, for the SONY A100 wound up being made by Tokina.
The 20-35mm range converts, after the SONY's 1.5x DCF, to 30-52.5mm. That's pretty much a wide angle to normal stretch. It allows focusing as close as 10-inches!
It is quick as anything else out there and relatively cheap on ebay. ($30-50)
While it may not have the overall range of the 17-50mm f/2.8s, this one will definitely get you close without the extra $400 expense. You'll be amazed at how nice this lens handles and delivers. It's definitely a lot brighter than the 18-70mm kit.
Here is a side-by-side between the two ... using identical settings ... I did it twice to be sure. They are merely cropped and resized, but otherwise straight from the camera.
EXIF data for both shots: 26mm - f/16 - 1/125 sec - ISO400 - 0 step EC - handheld
Tokina 20-35mm f/3.5-4.5
SONY 18-70mm f/3.5-5.6
The only thing that could have changed was the information being sent to the SONY external flash ... otherwise ... identical. Remeber, the new DSLR lenses have eight contacts where the older Minolta 35mm-film style only have five.