I've prepared a rather large image. Click through the following to get to Flickr where you can download it in full-size. This will then allow you to compare the Pentax K20D to the Olympus E-P1. Both cameras are two generations old; the Pentax system has seen substantial improvements to high ISO performance with the K-7 and then the current K-5 models. The Olympus cameras have seen only minor improvements through the recent E-P3.
These images were prepared by shooting a 20 pound note on my wall at the same distance, using a tripod and timed release. I used the same lens, the Pentax FA 43 Limited, mounted using an adapter in the case of the E-P1. The different sensor sizes result in different magnifications, but correcting for these would introduce a false element to the comparison, so I let them be.
The lens has excellent acuity in the middle of the sensor, almost unsurpassed in fact. This peaks at f/4 and f/5.6 so I used f/4 for all shots. This avoids any diffraction effects, which should kick in at f/4.5 on the E-P1 and f/5 on the Pentax K20D. (In fact the similarity in pixel density on these two cameras makes the comparison even more interesting.)
The RAW files were converted to DNG and processed in Adobe Camera Raw, with minimal settings (no noise reduction or enhancement). Some prefer to see images without any sharpening but I consider that all RAW files require sharpening to reveal the detail within. I set the Amount to 50, Radius to 0.5 and Detail to 30.
An automatic exposure curve was then applied to each image in Photoshop. No further processing was performed, even though output sharpening, colour balance, etc. would no doubt be required in a real world case.
The portion of the image you see here is a 100% centre crop, measuring 400 pixels square. For the Pentax this is a mere 1.10% of the total image captured. For the Olympus it is a slightly larger 1.31%.
As a further step I decided to apply Noise Ninja software to the ISO 3200 images in each case. Using the default settings the images cleared up nicely, with some reduction in detail, naturally. One could follow this step with a slight sharpening -- usually an appropriate step, though all image-processing is dependent on the subject and how you wish to represent it.
First, there are obvious white balance differences. I made no attempt to adjust for these. There are also very slight exposure differences. These are very difficult to eliminate entirely but they make a significant difference in image analysis, so beware!
The Pentax has more detail at all ISO values, with both greater macro-contrast and micro-contrast. Observe the eyes, the hair, the engraving lines on the forehead, etc. Noise is appreciable and too much for the finest uses by ISO 1600. At ISO 3200 the Olympus has distinctly less chroma noise. However, both images clean up nicely.
The Olympus ISO 6400 is mostly a gimmick, but might work out OK for a moody black and white conversion. One can also enhance the Pentax range to ISO 6400, by turning on a custom setting. I didn't bother, since my experience is it's similarly unusable.
In any case, one can "fake" even higher ISO values by underexposing the shot and compensating in development, "pushing" as we used to call it. Native high ISO settings only make sense if the camera can truly handle them without obliterating the image. Unfortunately marketing pressures push these limits higher and higher each month. Where once the megapixel race drove sales, the high ISO race is currently where it's at. I definitely prefer a camera that retains detail to one that removes detail along with noise. At the very least I require that these settings are entirely under user control, so that built-in noise reduction can be turned off completely.
My preference is definitely for the Pentax images, though there is nothing here that would stop me shooting with either camera. ISO 1600 is usable if one gets exposure exactly correct and fills the sensor with the subject. Otherwise I would stay at or below ISO 800. This conclusion comes not just from these tests but much experience in real-world shooting.
Pentax has a native ISO 100 whereas this is "pulled" from ISO 200 on the Olympus. Advantage Pentax, though I would prefer having ISO 50 available as well.