If you are working with a software package that has a separate "preview" function you can put the preview on one monitor and keep working on your main monitor.
Anyway, one big monitor and Spaces works great. I put Safari and Firefox in one space. Mail in a separate space and Dreamweaver in a third space. Whenever I hear the "bing" for new email I just click on the Mail icon in the dock and it switches instantly. If I click on a link in Dreaweaver, then Spaces switches automatically to Firefox.
Using Defaut-X I can get into any folder from any space.
I am working faster on the Mac even though my "Rocket PC" has a Quad Core processor, the fastest Intel makes, the main hard disk is a Raptor (10,000RPM), a super fast video card and 4MB of RAM. I put 10MB of RAM in the Mac Pro, ordered it with an upgraded nVidia graphics card so it's probably as fast as the "Rocket." One thing I can't quantify but I can see is that the MAC's OS-X Leopard does everything much faster than Windows XP Professional.