Be interesting to see what that video card is outputting before it loops back into the game board.
We have one of these machines, but it's on location, so I can't futz with it now. But I have come across some different games over the years that use a PC based video card and output in VGA, which then gets downconverted to CGA or EGA on the game board or an adapter board. I suspect with this game the video card is simply outputting cga or ega, whichever is selected. Still, like to plug a vga monitor directly into the video card and see what happens...

Anyway, in it's native form, NFL Blitz 2000/NBA Showtime on NBC shipped expecting a medium res monitor, but as noted above it's switchable to standard res.
BTW, you did get a hard drive with your boardset I hope. Your boardset picture doesn't show the drive and the boards won't do you much good without the hard drive!
D