Mecha, it is comparatively expensive compared to a hot dog but far more nutritious... I'd go for the OED on disc for the endless rewards of delving into (as Helen says) gold-standard research. It can be run in the background and called up as a context-sensitive link to any highlighted word in whatever other program you're running.

For a handy print version, any of the leading houses will give a good quick reminder; but nothing touches OED for sheer interest to the real language maven.