Progressive enhancement uses web technologies in a layered fashion that allows everyone to access the basic content and functionality of a web page (regardless of browser) and providing an enhanced experience to those with better bandwidth, more advanced browser features or more experience.
- Stackoverflow.com Wiki
Design for accessibility, like you should, and give IE8 users the most basic experience. You’ll be fine, and the Web can move forward.
It's often difficult to put hard numbers against the cost of not doing progressive enhancement and the financial savings of doing things the way we almost always do.
Progressive Enhancement is not a new technique; in fact, it’s built into the very fabric of the web’s original design, and ensures your website works everywhere — not just on Opera Mini and other proxy browsers.