well in the order you listed them their going from worse to best in ascending order.
Nvidia will make a gfx card, but in order to cover a broader market spectrum, theyll make different models of the same card. Differences between individual models such as between the 7800gt and 7800gtx can be between the number of pipelines and memory clock spped and core clock speed. Not every card that nvidia makes has a LS, GS, GT, and GTX model but most if not all have at least 2 of the variants.
It's basically to make each card avaialble in different price-ranges, to broaden the marketing of each card and create more price ranges to be more competitive to other gfx card manufactures (ATI).
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