Do you mean a pci express 1.0 or 1.1 x16 slot?
Which model is your motherboard? Let me know and I'll check which version it is.
With a few ifs and buts MOST 2.0 graphics cards will work in MOST older versioned slots. I myself am running a GTX 560 Ti in a pci-e 1.1 x16 slot.
All it means is that less bandwidth will be available for the card, that being said graphics cards barely fill the bandwidth of a 1.1 so it makes almost no noticeable difference to performance running a 2.0 card in a 1.1 slot.
With pci-e to the best of my knowledge if it fits in the slot then it works.