as i need a set up so i can have 3 monitors 2 lcd monitors and one set up for my tv.
If you want to run all 3 at once then you can't turn crossfire or sli "on" - turning crossfire on will put the power of both cards into one monitor, improving performance, if you want to run more than 1 monitor then you can't use crossfire - you can have the 2 cards running the monitors fine, but you can't enable crossfire.
If you want to run all 3 monitors at the same time then you can use any other ati card in the second slot (you could use a nvidia card in the second slot, but could face driver issues). Obviously if you want to have the option of running 3 monitors, and want to have crossfire for one of them (with the other 2 turned off while gaming) then the extra memory on the second card won't be used, but if you want to have all 3 monitors enabled all the time then you can get almost any other card and it would work and use all of it's power for the monitors it runs.
Just so we are clear on what different features are (i am very tierd atm, sorry if my post makes little sense) crossfire is when you combine the power of 2 graphics cards to improve the fps on one monitor, you can't have more than one monitor running while crossfire is enabled (it is easy to switch crossfire on and off in the catalyst control centre and enable other monitors when you need), but if you want to have a 2nd card to be able to use more than 2 monitors in your computer then if you got a 512mb card, it would be able to use the 512 mb of mem, unlike in crossfire where the 256mb card would mean that the 512 would be limited to 256 also when in crossfire mode. The cards would work, but if you want to use crossfire then getting the 512mb version would be a waste of money, it would work, but it would be a waste.