Going off this.
If you add up the costs for a server (bare minimum)
mobo- $140 (if you know cheeper then this with 4 pci express then let me know)
PCIe x16 Extender Cable $12*4
(4*440)/1353=1.3008 megahash per server cost
going with 6950 (http://www.newegg.com/Product/Product.aspx?Item=N82E16814150523
(going with 420 because it seems reasonable from the mining hardware comparison not sure what a normal megahash is for the card.)
(4*420)/1253 =1.3407 megahash per server cost
according to this site (https://spreadsheets.google.com/spreadsheet/ccc?key=0AsSwOT3E1XTGdGtpa1BQcmJLN2x6TG1MRmxzb29BeFE&hl=en_US&authkey=COWC8toK#gid=0
power consumption is basically the same as well a little in favor of the 6950.
At bare minimum the 6950 is better then the 5870 at those prices.
Let me know what the average megahash for a 6950 is because that will change the ratio a lot.
edit: found a better costing card for the 6950