While buying a Graphics card, check the following parameters 1st:
What generation of card it is, Number of stream processors in it, Memory bus width, Type and the speed of memory the card is using and the size of the memory.
1. GPU or Graphics Processing Init: GPU is a special purpose processor designed to process the graphics intensive task. Normally it consists of several number of very small processing units called stream processor. Normally in the same series (like nVidia 200 series), cards having more number of stream processors are more capable cards, considering other parameters. For example a HD 5770 consists of 800 stream processors whereas HD 5750 consists of 720 stream processors and both are having 128 bit memory bus and GDDR5 memory. Among these two, HD 5770 performs better due its higher number of stream processors.
2. Generation of GPU: Obviously a new generation GPU is more advanced than their previous generation counterpart. For example GTX 460 1 GB which is a middle end card is better than its previous generation highest end card GTX 285 due to its advanced architecture.
3. Memory Type: Normally standard graphics cards use GDDR or Graphics Double Data Rate memory which in most of the cases are confused with DDR memory. GDDR memory is specially designed memory module for Graphics cards and they are generally faster than their normal DDR counterparts. For example GDDR3 and DDR3 are completely different and GDDR3 data transfer rate is far higher than normal DDR3 memory. The current standard is GDDR5 (not DDR5) which offers twice the bandwidth of GDDR3 memory. So a GDDR5 running @ 500 MHz is actually equivalent to 2000 MHz standard memory speed. In cheap graphics cards like 9500 GT sometimes vendors pack a high capacity but lower performance DDR2 or DDR3 memory, not the GDDR memory to create marketing hype. A 9600 GT card with 256 MB GDDR3 memory will perform better than a 9600 GT Smart 1.5 GB DDR2.
4. Memory Bandwidth: It depends onMemory Bus width and the speed of the memory. Memory bus is the bus which connects the GPU and the memory of the graphics card whereas Memory speed is the speed at which the graphics memory is running. So more the Bus width or the memory speed, more bandwidth will be achieved. For AMD the standard Bus width is 128 bit for the lower-middle and middle end cards (HD 5670, HD 5770/6770, HD 5750/6750) and 256 bit for the high end cards (like HD 6870, HD 6950, 6970). nVidia uses variable bus witdhs like 128 bit for the middle end cards like GTS 450, 192 bit for GTX 460 768 MB, 256 bit for higher middle end cards like GTX 560 Ti, 320 bit for the higher end cards like GTX 570 and 384 bit for the highest end GTX 580. But higher bus width also makes cards more expensive.
Say G1 is card 1 is having 2X bit Bus and Y MHz speed for the memory
G2 is card 2, having X bit of bud width with 2Y Mhz speed for the memory
So roughly G1 and G2 will have same memory bandwidth, while G2 be cheaper as running memory in higher frequency is a cheaper implementation than doubling the Bus width.
So while purchasing the card, check the memory type, clock speed of the memory and the bus width. The combination of these three parameters will directly impact on the card's performance.
5. Size of the Graphics Memory: How much memory is needed...it completely depends upon the processing power of the GPU and the resolution at which games are getting played. For example a 9500 GT 1 GB does not make any sense as the GPU of 9500 GT is simply not powerful enough to use 1 GB memory whereas a GTX580 1.5 GB suffers from memory bottleneck as its GPU is very very powerful and thrives for more memory.
It is observed that upto 1920 X 1080 resolution 1 GB memory is okay with a standrad graphics card like GTX 560 Ti or HD 6870. But for more powerful cards like HD 6970 or GTX 570, more than1 GB is recommended.
6. GPU Clock speed or Core Clock Speed It is the speed at which GPU of the card is operating. More the speed, more performance you'll get. So in a same series say GTX 560 Ti, factory overclocked cards will perform better than the stock versions.
