What you are rendering seem to make more difference than what you are rendering it on. Using RayTracing in either your shadows or reflections will really slow the machine down. your graphics card does help but mainly on the high-res preview.

I use a P4 3.8 Ghz with 3 gig of ram.

I create and then render a great deal of complicated stuff. Some of the renders can take as long a 12 hours. Poser is a great program but can render slowly depending on what you want.

I tend to hit render and go to bed, I then check it in the morning, sometimes it is still rendering so i check it when i get home from work. Most times I find it takes about 1 hour.

As for comparing OSX to XP it is mainly the CPU that makes the difference. The Motorola CPU's (G5 and earlier) are geared for graphic render so they actually are faster even if they are rated at the same speed. AKA both are 4ghz. The new intel macs clock about the same as a Windows PC as they are now using the same chip.