We don’t know the full extent, but we do know this;

The most conspicuous evidence of Iraq and al Qaeda cooperation is al Zawahiri himself. Al Zawahiri who, with bin Laden helped issued the fatwa against the United States, was in Afghanistan with al Qaeda fighting the US and was injured. He fled to pre-war Iraq and has subsequently led the insurgency against the Coalition and the new Iraqi government. Al Zawahiri was no stranger to Baghdad. He consulted with Iraqi officials during a visit in 1998 and received $300,000 for his efforts. It takes an ideologically-blinded eye not to see at least professional terrorist courtesy between Iraq and al Qaeda in al Zawahiri's Iraqi visit, his refuge in Iraq after fighting in Afghanistan, and his present leadership of the Iraqi insurgency.

Taken from here

I am sure you think life was just great under Saddam. Getting murdered for speaking out against the dictator. Or having his sons rape and murder your 12 year old daughter.