My current video card (9600GT) is dying and I'm searching for a new video card. Between acquiring my current one and now, I got a lot more knowledge about hardware and I want to use that to pick my new card. So I decided to not just buy some popular card blindly, but to search for a card able to handle my hardware requirements.

I searched the specs at the NVidia site for the GT640 and was confused by the memory section and some questions raised. My current card's memory bus width is 256bit and has 1GB of memory. I checked Google about the importance of bus width. And all the links basically said the same 'The higher the number the more potential simultaneously traffic can be transferred'. This was already clear to me, yet there are currently a lot of new cards which are considered better than my current one with a lower bus width.

To go in more detail about my question I copied the memory info from the NVidia site:

                          GT 640        GT640 GDDR5
Memory Specs:
Memory Clock              1.8 Gbps      5.0 Gbps
Standard Memory Config    2048 MB       1024 MB
Memory Interface          DDR3          GDDR5
Memory Interface Width    128-bit       64-bit
Memory Bandwidth (GB/sec) 28.5          40.0

What puzzled me is that the Memory Bandwidth seems to me the most important part, yet the lower bus width has the higher 'performance'.
Is this due to the fact the memory interface is GDDR5 and is therefore able to have a higher memory clock speed (5Gbps)? If I am to buy a new video card, should I check the bus width? Memory clock? Bandwith? Amount of memory?

My current card ahs 1GB memory, so I was searching for a 2GB memory card, but now I'm not so sure any more whether that is really 'better'.

My main question:
To me it seems that memory performance is made up by the combination of bus width and frequency. Is this true? If yes, why are there so many sites telling me I need to get a card with a high bus width? If no, then what IS important when it goes about memory performance on a video card.

NOTE: The memory bandwidth is (almost) never displayed on vendor sites. How can I determine which card is better without knowing the bandwith?