DS 6.9 also supports x64 bit arch as server.
But if you have installed ITMS 7.1, you can prefer to use Ds 7.1 which is a part of ITMS. It does nhot contain all the functionality but will definaltely be replacing DS 6.9 in future.
Personally i will not prefer to install DS 6.9 and ITMS (with dattabase engine) on same machine.