two 16 channel cards in 1 pc?

[quietas]

I'm using two of the 8 channel PV-981, each channel is about 2.5fps.

From there you lose frames each time it changes from one came to the next. With four cameras you will lose at least 4-8 fps from what I have seen. I suppose it all comes down to how many FPS is needed for your application.

Do you need full 30fps live video?

If so, you'll need a vast amount of storage and higher priced cards. Storage is a concern. at 2.5fps on record at 320x240 in color, I use 2tb per 40 days or so for my 16 cameras.