pito:
You need to do this calculation in 64-bit integer arithmetic (which makes using the Due look better!)
This works on the Uno:
unsigned long long DDSword, freq, DDSclock;
DDSclock = 125000000LL;
freq = 50000000LL; // required freq
DDSword = (0x100000000LL) * freq ;
DDSword = DDSword / DDSclock;
The above works 
You may do a check with:
Serial.println((long)DDSword, 16);