Logitech control center is complete crap. Yes, it will let you assign each button, but only on a GLOBAL basis. I have a MX510.
I highly reccommend USB Overdrive. It's very easy to use, and will let you assign every button on your mouse on a per-application OR global basis. Each button can be assigned as a click, a certain type of click, scroll, volume, keystrokes, and tons of other options. If you can't do it with USB Overdrive, you probably just can't do it.
SteerMouse mentioned above might be just as good...never heard of it until now.