Refitting: put it out and plug it back in. Make sure that you have all pins inserted firmly in the PCI slot and that you can't see them sticking out of the slot anymore (the sign that it's put into place well).
PCI 3 is backward compatible with PCI 2. In theory PCI 2 doesn't support the same bandwith speed as PCI 3, but as far as I know most video cards barely fill out a PCI 1 bandwith. The only possible difference you could feel is stuttering (PCI 2 slot -> old motherboard -> old chipset -> might bottleneck your system, which means slow it down).
Try refitting the new card; try putting back the old card. Also make sure there isn't any grease on the new card's pins (fingers get oily minutes after washing your hands because we have glands that secrete oil all over our skin). If you have grease or fingerprints, then clean it with a cloth. If none of these work, try the new card in another PC. At the very least your old card should work. I still think it's not plugged in well.
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