Doing tons of CSS stuff lately and really loving it. I have found some supreb reference sites:

• AlistApart
• IE7
• StuNichols
• W3CShools
• Web Standards
• CSS Zen Garden(Zen Garden Interface Tool)
• Jeffrey Zeldman (his book rules)
• MezzoBlue
• Flumpcakes CSS Optimiser
• Web standards CheckList
• CSS Destroy
• CSS Play

There are probably tons more but these are class.

From a coleague in work, check out http://www.webxact.com/ , a superb tool for seeing what's slowing down your site………… flash usually.