For MD5 you don't need bus bandwidth as all work is done one video cards itself. On this forum there is even explanation why for hashcat pciex1 is more than enough. Don't be lazy and search a bit.
p.s. 1xHD6990 gets around 10G MD5 on oclHashcat-lite (not over clocked) so you will need as much as 4 of them to achieve speed you want by hardly over clocking them and it's max number of possible AMD gpu.
p.s. 1xHD6990 gets around 10G MD5 on oclHashcat-lite (not over clocked) so you will need as much as 4 of them to achieve speed you want by hardly over clocking them and it's max number of possible AMD gpu.