Well heres an easy one. Try this, make sure your monitor is plugged into the new video card. You can turn off onboard video by going into the bios, or just going to the hardware manager. (Right-click my comp, properties, click one of the tabs, and click on hardware or device manager.)
Rig -- Emachines T5026, 3.06ghz P4, 300W Bestec PSU, 160 sata hd, 512mb dual channel 3200 ram @ 3-3-5-8, ATI Radeon 9250 pci, dual layer 16x dvd+/-rw.