DDIO clock timing

The best way to see if this is possible or not is to create a simple, mock-up, design and run it through TimeQuest.

But the maximum possible clock frequency is usually listed in the "DC and switching characteristics" part of the device's Handbook. The slower grades of Stratix III might not do what you need.

You should also read the DDR Timing cookbook:

http://www.alterawiki.com/wiki/ddr_timing_with_timequest