I've always used 800x600 and recently I've learned that if I use KDE under a really high res it looks so much better! But, does this slow down the cpu or take up extra ram? Or does it do anything derogatory? Thanks!

Yes, in a way it does. It makes your video card work harder but it is so little that you shouldnt notice it unless you have a real old system.

If you're using a system built in the last 5 years, and you notice a drag on your resources during normal use (3D games don't count), then chances are something's set up wrong. For 95%+ of people, everyday useage with any reasonable amount of RAM shouldn't mean much of anything. On the other hand, you might not get the best (or any--with cheap cards) 3D performance with a higher setting. Usually, the card will drop you into a better setting. Still, for the most part, it's better to have something that makes you happy than something that's crappy but saves a couple megs of ram. I run 800x600 only on the oldest monitors (the one I'm on at the moment is circa 1997). On anything newer, especially 17+ inches, I run 1280x768 or better, depending on my needs. If you run engineering/drafting software, you almost have to run high resolutions, just because most of the software is set up for them.