Hello! My computer has a PCIE 2.0 slot for the graphics card. I know this because I checked on Nvidia's web site and under the GTS 240 it said PCIE 2.0. I then checked Best Buy and the graphics card I wanted to get needs a PCIE 2.0 x16 slot. Will my PCIE 2.0 slot work with the PCIE 2.0 x16? If anyone can explain this to me I would really appreciate it.
The short answer, though, is your PCIE 2.0 is most likely x16....as long as you're running a single card. If you were adding a second card, you would most likely get 8x out of each slot unless it was one of the higher end double x16 boards.
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.