I never saw Kasz so angry like this lol xD

Yeah, was in the middle of two 11 hour shifts, and it just pisses me off on how you basically have to find benchmarks because people have decided to use completely made up numbers in their titles rather then anything helpful.

Half the time you can't even judge by number progression because an earlier series version could get something built better.

You don't really need a good GPU for a game like XCOM. CIV should also be more CPU dependant.

Hell my 400€ Laptop I bought last year, can easily play Bad Company 2 and similar games on medium settings.

I have a HD5470 btw.

http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units
http://en.wikipedia.org/wiki/Comparison_of_AMD_graphics_processing_units

Rough approximation, performance = shader count * clock speed * prefactor
The prefactor is different between pre-5000 AMD cards, post-5000 AMD cards, and Nvidia cards.

Those are all very old card series', all pre-2008 I believe.

All newer ATI cards (Radeon HD) go up in 1000's for series, 100's for large power increases and 10's for small power increases. The X400 series are usually low end, X900 series high and X500 - X800 bridge the gap. XX70 would be slightly more powerful than XX50 and so on.

It's a similar system for the Nvidia cards, but with only three digits. They go up in 100's for series and 10's for power.

Mobile card use the same numbering scheme I think but have an "M" on the end and are less powerful than their desktop counterparts. So the GTX 660 would be better than the GTX 660M.

ATI are currently on the 7000 series and Nvidia are on the 600 series.

EDIT: Did I answer your question right? Not quite sure what you need to know.

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.