Provide a text only option for people who don't want to wait for graphics. Most people will be able to shut off graphics using their browser, and you should test the readability and usability of your site with the graphics shut off. The text only option is also appropriate for users with sight impairments. Keep your messages and other text short and to the point. Most users will not read long passages on the web so your information must be meaningful, precise, and very short.
Headers can be larger.
Arial, Times Roman or Sans Serif tend to be the easiest to read. Arial seems to be the most popular web font. Always provide help on all of your pages. Users will not stick around to figure out problems on their own. Borders, rules, bullets, etc. help to organize your page and make it easy for your users to navigate.