There are different kinds of PCIe 2.0 slots. PCIe 2.0 x16 is what graphics cards use. PCIe 2.0 x1 slots are a lot shorter and they are used by things that don't need as much speed like sound cards and wireless cards (and many others). PCI slots are older and a different shape. I found the image below in a quick google image search and it shows PCI, PCIe 2.0 x16 and PCIe 2.0 x1 slots. It also shows a PCIe 2.0 x4 slot but few motherboards have those these days.
It looks like GTS 240's use PCIe 2.0 x16 so if that card is currently in your machine then any PCIe 2.0 x16 card should fit in the same slot.