you'll be able to run it on Low Quality 640x480 (which still looks amazing btw). But that's about the highest level I'd predict you'd be able to run it.
a PC with:
1.5 Ghz Pentium 4
512 MB RAM
GeForce4-440 64 MB RAM
gets an average of 30 FPS at these settings.
If you want to run it at higher resolutions and quality you're gonna need to buy a card with a larger amount of video RAM (256 recommended otherwise settle for 128)
Doom 3's textures are HUGE. Carmack has even stated that to run the game on Ultra Quality settings, you need a 512 MB video card (does not exist yet) because all the textures are uncompressed.
ON progressively lower video settings, textures are compressed more and more to accomodate for the existing video cards.
low quality itself requires a 64 MB card to run relatively smoothly
medium quality requires a 128 MB card to run smoothly
high quality requires a 256 MB card to run smoothly
and i already told you about ultra quality
There's still a long way to go before the game is fully optimized for Mac OSX but I expect the same high requirements to exist.