The Quadro NVS 450 actually has two GPUs on one board. What's most likely happening is that the X server is probing a GPU that doesn't have any displays connected to it.
Try either of the following:
1) Explicitly set a PCI Bus ID in your X configuration; for example, your X log shows that it's trying the GPU at 4:0:0, while your lspci output shows a GPU at 3:0:0 and another at 4:0:0. You can try adding the following line to /etc/X11/xorg.conf:
2) Try other ports on the board.