If these cards were $50 I would buy one. Plus, who wants to add another fan to their rig? After dropping $300 on a new video card, who's willing to pay an extra $150 for physics? Sorry Ageia, in order to sell something, it's gotta be attractive to the consumer.
They're barking up the wrong tree with standalone cards... they need to get their chipset integrated into a video card to have a prayer long term - it's the only way they'll get enough market penetration to entice game makers to start writing code for their chipset.