I need help figuring out how to speed up my render time on my Mac Pro:

2x2 GHz Dual-Core Intel Xeon
2 GB 667 MHz DDR2 FB-DIMM
NVIDIA GeForce 7300 GT 256 MB VRAM
Poser 7

I have a large animation project for work, and have found that ambient occlusion is necessary for the quality of the images we're looking for (need shadows on the teeth, otherwise they're too bright, etc.). However, the end product will be displayed in 640x480 streaming video, so texture quality isn't a huge issue.

My problem is that it's taking way too long to render the sequences, about three minutes a frame, and I need to find a way to cut it down. Getting a second computer isn't an option, and as far as I know network rendering isn't available in Poser yet. I also understand that VRAM has no bearing on how fast Firefly renders, correct?

So what tips could anyone give to help me out? Is there another hardware upgrade option that I haven't thought of? Is there a third-party renderer that I could use that would be easy to import my scenes in, which will render faster? Any settings I could chuck that won't make a huge different in a compressed video format?

And please...no "you shouldn't use a Mac" speeches...I've had enough of those already. PC simply isn't an option.  :-P

edit: Forgive my newbishness as well, I'm still learning how all of these settings work and the amount of options is quite daunting.