The projector will have a different display.
If that's the case, then all that needs to be changed is a second video card from this list
will need to be purchased, and only one DVI splitter
is needed. Then, connect the 3 TVs which will be showing the same thing to one video card (using the DVI splitter), and connect the projector to the other video card. This will allow the projector to have a display independent of the TVs.
Do you think a consumer card would be powerful enough to display on 50in TVs at a reasonable resolution"
Unless the church will be rendering 3D video in real time (ie. playing 3D games or using 3D modelling software), any consumer video card on that list will be more than powerful enough to show videos or powerpoints on virtually unlimited monitors (only limited by number of connections available). These activities really don't even involve the processing power of the GPU or video RAM (though some GPUs will decode HD video, that task can always be done by the CPU instead).