System requirements for new games are always nice to check into before you purchase the game, eh?
The OP has an iMac (maybe a MacBook Pro?), I think.
I will disagree with ElDiableConCaca on video card upgrades. Outside of the various Mac towers, few Macs have any possibility of upgrading the video. A failed video card almost always means replacing the logic board. Even those iMac models with replaceable cards don't have upgrade options available, so upgrading the video in most Macs is impractical, at best, and certainly not easier than on a PC tower.
The only available upgrades for Macs are for some PowerMac G4s, and (quite limited) for G5 towers. Again with the MacPro tower, some video upgrades are available, and simple to do. There's likely more options now than on older PowerMacs.
Not for iMacs. Pulling out the logic board to replace a video daughterboard is not my idea of an easy task.
Anyway, the X1600 would be supported, but appears to need Leopard (OS X 10.5) or higher to use the game, as EDCC suggested.