Compared to...

Below you will find a studio comparison between the Canon PowerShot A510 and the Casio Exilim S-100 at ISO400.

Studio scene comparison (@ ISO 400)

• Canon PowerShot A510: Aperture Priority mode, ISO 400, Default Image Parameters,
  Manual white balance, +0.7EV Exposure Compensation
   
• Panasonic DMC-LZ1 : Program mode, ISO 400, Default Image Parameters,
  Manual white balance, +1.0EV Exposure Compensation
   
• Lighting: Daylight simulation, >98% CRI

Canon PowerShot A510	Casio Exilim S100
ISO 400, 1/640 sec, F5	ISO 400, 1/500 sec, F5
1,614 KB JPEG	1,600 KB JPEG
Noise, std dev: 4.0	Noise, std dev: 4.4

At ISO 400 the Casio looks a lot noisier, whereas the A510 actually has relatively low noise (certainly a lot better than the 4.0MP A520). Even so, I'd only use the ISO 400 setting if there was no alternative.