I was looking at the CSS on http://www.simplebits.com/ the other day, it's really well structured (try opening it up using the Web developer extension in FF) - was also interested to see that the IE hacks had been put into a seperate file altogether (http://www.simplebits.com/css/ie.css)
You really shouldn't have to have a separate CSS for any browser other than IE. Mozilla, FF, Opera, Konq. and Safari are all updated regularly enough that a user won't have an older version for much longer than they need to.
However, people still run IE 5.5+ so you need to use another CSS for them. The easiest way is with conditional comments: