Im about to go to the the way of the Mac. However, I have read a few things on line that the imac 20" (with 256mb graphics card) isn't any good for photography.
Actually, I recommend that you keep the computer off and concentrate on your photography by learning how to use the camera instead of fixing things with photoshopping.
It amazes me what some people own to do basic photoshopping, and then they may ask me, "How do you do anything?" I can use an old CPU with 512 MB of RAM and a graphics card with 4 MB of memory for image processing. These applications like Aperture and Lightroom are simply resource hogs. I tried Lightroom once and couldn't even get the stupid installer to run because it wanted more than 512 MB of RAM to do anything. Code streamlining seems to have become lost on many companies, and they've also lost my business because I only upgrade hardware to handle data that I produce, not excess CPU cycles and memory leaks that software developers produce.
The only intractable things that have become more demanding for me are the increasing sizes of the image files and the overall size of each job. Most of that can be handled by simply adding more RAM. A faster CPU and hard disc can also help, but only for lengthy operations involving many processing cycles. For amateur work, I don't bother with the computer much at all. It's easier letting the camera do the processing, and maybe do some batch conversions from RAW files, but nothing extensive for post-processing.
http://www.bibblelabs.com/products/bibble/speed.html
I like to push JPEG and RAW files through basic processing adjustments and send the files to a batch queue for output. That's less time wasted sitting in front of the computer screen for me. Bibble handles that automatically.