NVidia traditionally has a rather bizarre naming scheme, so if it confused you, you're definitely not the first. It'd make more sense if they went by Intel's scheme and sorted cards like... let's say... "Geforce MX 1000" for an old integrated card, all the way to "Geforce GTX 9450" for a somewhat newer top end card.
Anyways, "GT", "GS", and "GTX" are prefixes that supposedly classify the general tier of the graphics card, but they shouldn't be considered unless you have multiple models with the same number.
As for suggested cards, almost anything from the last two years will run Starcraft 2 (I should know, since I'm running on the Intel GMA 4500MHD, which is terrible for games). Personally, I'm going to recommend the ATI Radeon HD 5570, which has reasonable performance, supports the latest technologies, and also is fairly small and power efficient. I don't know how easily your computer's case is opened, but so long as you can get it in there, you'll see a great boost to SC2.