Top producer is pretty much an online application now. That should make is cross-platform, just depends on what browser you are using. If you are only using Safari, try out Firefox and see if all features are supported. You can download the Mac version from this link: http://www.mozilla.com/products/download.html?product=firefox-3.0.5&os=osx&lang=en-US Or, pick the version you want from the list on this page: http://www.mozilla.com/en-US/firefox/all.html
In all honesty, I do not look to a Mac for business related tasks but anything that is web based should work no matter what operating system you use. On another note, I do hear that the address book that is built into the Mac is fairly adequate.