A credit card is useless for spreading thermal goop: it's a flexible edge which will NOT provide a consistent thickness of goop. Thickness is important. Doing it wrong can result in temperatures 7-10 degrees higher than necessary.
I love the way you guys say "Don't put on too much!" but you don't say how much is too much. Honestly - what sort of advice is that? At Digg I would say "Lame! Buried!".
The spreader of choice is a straight razor blade - you know, the one with the reinforcing strut down the back of it.
The amount to use is a blob about the same size as a grain of rice, for a GPU of that dimension.
However, there are other factors in play, the surfaces must be 100% clean of lint and finger grease - which can cause major defects in the Specific Heat capacity of the thermal joint.
To clean the surfaces you need Isopropyl Alcohol (IPA) at better than 99% pure. Having a lint free cloth is also desirable.
After cleaning, I put a plastic bag over my index finger and a very tiny amount of thermal goop, and I rub it into each surface, and then wipe it clean again. This fills up the hollows in each surface, something which doesn't happen at all if you just throw some goop on and spread it around. Removing all the possible air pockets from the surfaces is very important.
I **THEN** begin the process of placing the goop, spreading it, getting it even, and installing the HSF.
A great set of instructions is to be found here: http://www.arcticsilver.com/instructions.htm