Your card supports pixel shaders...albeit not the best card to be running Halo with.
Hold down command (the apple key) while starting up Halo, and select some pixel shaders. NV path would be better for you, it looks not quite as nice actually, but it will run faster, and your cards not top-notch. Make sure FSAA is off. Model detail can be high, and if you want lens flare make sure you have the latest update, otherwise it will slow you down bad.
If performance is bad, turn off shadows, turn off specular, and bump down your resolution. But it will look a LOT prettier, and should run acceptably at reasonable settings.
If you want a much better card, for about $150 or a little less you can get a PC Radeon 9800 Pro, which is very, very simple to flash to Mac. If you need help with that or want more info, let me know.
wangahrah