I also don't understand why you need a card with 256 bit bandwidth with exactly 4GB of memory. A variety of factors affect graphical performance, gpu architecture, core speed, amount of memory, type of memory, memory bus, memory speed, shader processing units, drivers and much more, you should buy the best performing card in your budget. You can specify what you want your card to have and that's your prerogative, but it doesn't make sense to us and geoff is trying to help and explain why.
It would be like me going into a car dealership and saying 'I want the fastest car for less than 20,000, but it needs to be diesel and have a 3 liter engine'.
The dealer says 'the fastest car we have for under 20,000 is a 2 liter turbo petrol that goes 0 to 60 in 5 seconds and does 170 mph, which costs 16,000'. He then goes on to say 'we do have a 3 liter diesel that costs about the same but its slower, 0 to 60 in 7 seconds and goes only 150 mph' and you say 'go on then I will take the slower diesel'.
It maybe that in your budget a 4gb 256 bit card is the fastest card, it may also not be, a different card with different specs may perform better for at a similar price, so why not go for the better card it you can.
