The new Intel Sandy Bridge CPU's come with integrated video that can do 3D blu-ray. Also, radeon HD5xxx and HD6xxx series cards do blu-ray too. For Nvidia, the 4xx and 5xx cards support 3D blu-ray, but not all of them bitstream HD audio. You'll have to do your homework to find the ones that do (GT 430 does BTW).
You don't need multiple cards for 3D blu-ray. You just need one connection using HDMI 1.4a. The cheapest route is probably a Sandy Bridge CPU with an H67 chipset motherboard.
Edit- BTW, check out the mini-ITX build I recently completed in my sig. It may give you some ideas.