When I create a Base System Builder design for the SP601 and SP605 boards, it selects 800 Mb/s for the Memory Controller Block (MCB) clock, but the data sheet says that the MCB can only operate at 667 Mb/s.
To avoid this problem, modify the output clock of the Clock Generator to 666.6 Mb/s:
In your XPS design, right-click on the Clock Generator instance and select Configure IP.
Change the MPMC_Clk_Mem_2x from 800 to 666.666.
This problem has been fixed in the release of ISE 12.2 software.