Is your website obsolete?
In case you need more motivation to convert your site to XHTML and CSS, Zeldman rants that "99.9% of Websites are Obsolete."
Peel the skin of any major site, from Amazon to Microsoft.com, from Sony to ZDNet. Examine their tortuous non-standard markup, their proprietary ActiveX and JavaScript (often including broken detection scripts), and ill-conceived use of Cascading Style Sheets - when they use CSS at all. It's a wonder such sites work in any browser.