http://www.blackmagic-design.com/products/intensity/

At $249, this seems like a great HDV output solution for previewing on an external monitor... unless someone can suggest something better? I'm looking to be able to preview video played from FCP 5

Should work fine for you if you are using a HD monitor with HDMI.

If not, just get another video card with DVI (or better yet if you have an unused port) and hook up a DVI monitor and use Final Cuts cinema desktop feature.

-brian