A question about CCD size

What are the quality differences between my A640 with:
1/1.8 " Type CCD + DIGICII

and a:
1/1.2 " Type CCD + DIGICIII

?

I'm primarily thinking more about noise in high ISO, and light... and how this all compares to something like a DSLR 400D.

Thanks.

Which PowerShot has a 1/1.2 sensor and DiGiC III? AFAIK, the 1/1.7 sensor in the 12.1 MP models is the biggest Canon has in anything. And without the pixel count, sensor dimensions don't say very much.

According to dpreview .. the S5IS
http://www.dpreview.co​m …050703canons5is​.asp#specs

edit..
EEP - read it wrong ... it says * 1/2.5 " Type CCD * - thing is, I don't know anything about sensor sizes, but am looking to upgrade from the A640.

That's a smaller sensor; yours is about 7.176x5.32 mm while the S5 has a 5.76x4.29 mm one. Sensor sizes. I don't think DiGiC III will quite compensate for the sensor being about 2/3 the size. And there's going to be no comparison with the 400D - it'll blow them away for noise performance (among other things).

So 400D is a good intro to DSLR then, up from the 640. Noise in images is what's hurting my live action shots the most. Sure there's software to combat it, but you shouldn't have to.

After a couple of years almost with the 640, I feel i've wet my feet and am ready to dive into the pool

Here are 2 good charts for comparing sensor sizes:

Funny . . . I'm just picking up a reconditioned A630 for the sensor size and SDHC when I'm traveling with multiple cameras, since the 1D3 takes SDHC and I've added a couple of SDHC-CF adapters so I can use the cards as extra storage on long trips. Couldn't find an A640, or new A630. The A620 will stay in my backpack for work.

I must confess, when I upgrade, i'll be doing the same and keeping it as a backup camera. Looking at the sensor sizes above, it's easy to see why the 640 has so much noise above 400 ISO though - I confess, i'm actually shocked ! - Thanke for posting those Msowson