What ever you do don't tell us what motherboard you have, that would make the answer too easy. Good news PCI-Exprees 2.0 video cards are backward compatible with pci-e 1.0 and the peformance loss is in the neighborhood of 5-8%. This should not keep you from buying your new card.
You can download cpu-z a free program and it will tell you which you have.
good luck