Video cards don't use "sockets", they use "slots". Unless you have a really old motherboard that uses AGP, all boards now uses PCI-E. For video, you need a PCI-E 16x slot (longer). There are various versions of PCI-E, 1.0, 1.1, 2.0 and 2.1, but they are inter-compatible (but performance might vary a bit).
Also note that very few boards will have no PCI-E slots at all (they use on-board video), but they are mostly for very small form-factor PCs.