Filter quality varies significantly between manufacturers. The type and quality of the coatings on the filters is one of the key areas influencing image quality in digital photography, as well as the glass used.
Here is photographer’s site with some tests from a couple of years ago that helps illustrate the differences between no filter, quality filter and cheap filter.
http://www.kenandchristine.com/gallery/1054387
And some more reading:
http://www.luminous-landscape.com/colum ... b-05.shtml
Concerning a tripod, a lot of the decision depends on the camera + lens, and your specific needs e.g. tripod weight, type of shooting, portability, etc.