You'll find that some settings have more of a performance cost associated with them than others. For instance, you'll likely be able to set your level of anisotropic filtering to maximum at very low impact, while enabling antialiasing at all can be quite draining. (WoW refers to antialiasing as multisampling. Choose 24 bit color, 24 bit depth, 1x multisampling.) I'd expect your video card to have no problems with texture resolution set to maximum, and it's likely that the only place maximum spell detail would cost you would be in a raiding environment. Your mileage may vary when it comes to the ground clutter options, but changing them doesn't require a restart so it's easy to experiment.
Make sure that level of detail is checked under the miscellaneous section as that's a performance enhancing option.
Edit: Oh, and one other thing. WoW's interface is highly customizable with addons, but some of them can impact performance quite dramatically. If you have many addons, it may pay you to examine their configuration. For instance, some unitframes have an adjustable update frequency. Once per second has a much lower cost than once each frame or even every 50 milliseconds. Some raid managing addons like CTRaid can be configured in such a way as to drag even a modern system to a crawl. The simplest way to check your addon performance impact is to just disable all of them and compare framerates.