The PCI-E 3.0 cards are backwards compatible, so it will work.
A PCI-E 1.0 slot has one quarter the bandwidth of a PCI-E 3.0 slot. For cards like a GTX 750 Ti or R7 260X, or for slower cards, this shouldn't be a problem. For high end cards, your processor and motherboard will be a bottleneck.
Thank you both!