Your current 6950 will do eyefinity, you just need to buy an active display port adapter to power one of your monitors with.
Like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16814999030&cm_re=display_port_adapter-_-14-999-030-_-Product
And I don't think it's possible to run 3 screen eyefinity AND a 2nd primary display at 1080p, certainly not with a single card or crossfire if thats what your goal is. You will just have to unplug a monitor and reconfigure your displays in CCC when you want to swap out and use the 1080p display I think.
If the 3 monitors are ONLY for work usage then eyefinity is not really what you want IMO, it's much easier just to extend your desktop onto the other displays, you don't need the toolbar going across all 3 screens and it's not like you're going to want a single excel document spanning 3 pages, if you're like me you'll probably have word on one and excel on another.
So basically if you are replacing the 1080p screen with the 3 monitor setup then just get a DP adapter and use your current card, if you want to run 3 monitors in addition to the current 1080p one I don't think eyefinity is an option, so just buy any ole cheap ati card to run 2 of the smaller screens and power the other 2 with the 6950.