I recommend Lightzone. They require registration before downloads, which is absurd. If you want to, you can avoid this by cloning and compiling from github repository.
Use case: photo manipulation, including color channels, hue, saturation, brightness, white balance, contrast etc. It is not Photoshop replacement. On commercial (and non-linux) side, feature set is comparable to Lightroom or Aperture.
Memory: on my computer, even with larger folders, 1GB of memory (for Lightzone, excluding OS and other programs) seems to be enough. I have Nikon D700, so my RAW images are approximately 25MB - I guess size/resolution of the original images affect memory requirements quite heavily.
Processor: more is better. As Lightzone is implemented with Java, it's not lightweight. I have 3GHz Intel i7, but it is a bit sluggish from time to time (this is highly subjective, though).
Compatibility: earlier I had problems with white balance on D80, but that is fixed already. I don't know about other RAW formats, but there's database available.
Usability: getting started takes some time, as everything is on toolbars, and some icons are non-intuitive. Additionally, some functionality - for example, exporting RAW files to TIFF - is a bit non-intuitive, but is still available.
File formats: supports exporting (converting) to JPEG and TIFF. Similarly, opens RAW, JPEG and TIFF files.