In a nutshell,
there are two main GPU makers, Nvidia and AMD (ATI)
NVIdia makes the "Geforce GT (number)"
AMD makes the "Radeon HD (number)"
the numbers for NVidia cards like 100 , 200, 300, 400, 500,600
meaning first generation , second generation , third generation .....etc
AMD goes for 1000 , 2000, 3000, 4000, 5000.
same meaning, they just use four digits instead of three, maybe to make them look better , I don't know.
so if you buy a NVIDIA Geforce GT 650 for example , you can upgrade to GT 660 , 670 , 680 , 690
some GPUs say stuff like GT 630 M , M here means Mobile, meaning this is a laptop GPU not Desktop ,its smaller sized, with no fans and such.
sometimes they say Geforce 560 TI , the TI here you can ignore, its just pointless, and it means Titanium, since most graphics are made from the same materials nowadays this is outdated,
Other terms to know about GPUs are
"Cuda / PhysX enabled" (for nvidia) / "OpenCL Enabled" (Multiplatform)
Cuda or OpenCL is a technology for rendering (like on Sony Vegas or Photoshop) , if you graphic card is Cuda enabled for example, that means it can work with the CPU to render things faster ,when you render stuff in 3D or when you render videos on Vegas it makes it faster, you don't really need it, but if your GPU has this , then its pretty cool to have, and you might use it , as for gaming it can be used for something called "PhysX" which is another one of Nvidia's gimmick , which means MAOR Objects flying at you!!!!!!! (look it up on youtube) , this is actually pretty cool in Borderlands, its the best use of it so far.
"SLI Enabled" (for nvidia) / "Crossfire Enabled" (for AMD)
SLI and Crossfire meaning that you can connect two GPUs together to double their power, for example if you have an 560 Ti with another 560 Ti and both of these are SLI enabled, you can have a connector (that is called SLI Bridge) and connect them with each other, I don't recommend this, as you might as well buy a better card, this is only for if you are processing too much stuff, as you will need lots of cooling, maybe liquid cooling too, if you're that hardcore, the SLI connectors come with the Cards so you don't have to buy them, if you get them , keep them.
