When I turn it on, the fan spins and everything seems to run...". Do you mean the floppy disk initialises, hard disk spins, and HDD l.e.d. flickers in sympathy with the probable loading up of the operating system?
Some motherboards have a jumper to select on-board video - check mobo manufacturer's manual.
Try completely clearing CMOS. Either use the CLR-CMOS jumper, or remove the battery for a short time.
Does this new mobo require the additional 4-pin 12v power connector?
Make sure replacement mobo is not shorting out on a redundant mounting pillar from the previous board.
Temporarily remove any cards plugged into the PCI slots, just in case of conflicts. If still no joy, then disconnect everything so that only the PSU is plugged in.
ROGER - G0AOZ.