Hi Gooshin,
Short answer:
yes.
Long answer:
All these things come into play (though probably least the GPU speed).
It depends a lot on what software you're using and what operations you're considering.
I'd keep the RAM and CPU speed high, perhaps compromising on the graphics card. But in addition to the things you've mentioned, consider motherboard clock rate (FSB, or front-side bus speed), hard-drive speed, as those things impact most of the large data transfers you will be doing with a K20D...
RAM will impact your ability to edit huge stitched panoramas, and make it faster when you have 10 photos opened. But then your hard-drive speed probably has just as much impact because your system will probably be paging your RAM at that point anyhow...
Adobe CS3 appears to make use of both CPUs when I'm doing a folder full of RAW->JPEG conversions, so I'm guessing that it might even make use of a quad-core. If your photo converter isn't written to take advantage of multiple CPUs, then having a quad-core won't really help that aspect, but it would likely make everything else faster while you were doing a conversion in the background.
Just as with a camera, buy the best you can afford just at the time you need it.
If you post the spec'ed components you're putting together, a pointed recommendation would be easier.
-Chris