[lode]

My hardware-dealer was kind enough to let me try out a setup and change it if it didn't work out. Luckily I had no problems. Both cards have Nvidia GeForce chips.
I highly recommend befriending a store and asking for the same deal.

It sounds like a good idea to get two Nvidia cards. Make sure the card that will handle video-out has no less than 256 mb.

My screen setup
I have two (old) vga flat screens connected to one gfx card. I use these for desktop, PT editing etc.
The other gfx card sends one DVI video and one composite video out. Both are connected to the Plasma screen on the AV ports.

The video window for PT is played on the composite video out. The gfx card detects the viedostream and plays it full screen on the DVI out. This is a hardware feature and from what I can tell does not burden the CPU.

The point with this setup is that I can split the composite signal so it is also sent to the voice booth for voice-overs. Works wonderfully for me.