How do I decide which filter to try?
As you've seen, you can't always judge a filter by its name. If you want an "art" effect, decide first whether you want full color or limited color. For the latter, look at the Sketch filters. Consider how abstract you want to get.
Cutout and Conté Crayon are both successful with most pictures.
Is there a way to tone down a filter that does what I want, but does too much of it?
Yes. Near the top of the Edit menu is a command called Fade. It enables you to change the strength of the filter from 100%0%.
Are the filters that come with Photoshop 7 all there are?
Nope! There must be literally thousands of filters that have been created by individuals or companies. You can locate them by searching for "Photoshop filters" or try the following Web pages:
http://www.flamingpear.com/blade.html (This site has several awesome sets of shareware plug-ins.)
http://dir.yahoo.com/Computers_and_Internet/Software/Graphics/Filters_and_Plug_ins/ (This site links to many pages of filters and other useful Photoshop goodies.)