[eluser]nelson.wells[/eluser]
This is more related to HTML and CSS than CodeIgniter. CodeIgniter is the server side language, the design of the page is client side. You mention nested HTML tables; you need to learn how to use more modern HTML and CSS to do layouts. I like [a href="http://tizag.com"]Tizag[/a]'s tutorials.
Also, as far as converting a .psd to a website... slicing is the way. Depending on the design, some styles (like shadows) can be emulated with CSS3, but for more complicated things slicing is the way to go.