Personally I don't do the credit card trick, I prefer to use thermal pad on the RAM chips, that do the same trick but with better perfomance. The idea is that either the credit card trick or the thermal pads on the RAm chips are used to ballance the pressure from the x-clamp fix. But, if you not tighten too much the screws on the GPU heatsink and be sure your heatsink is parallel to the mainboard, not bending it, and still touching your GPU, then no credit card or thermal pads on RAM chips are needed really. I just use them to avoid E74 or freezing issues for the future.
Now, the thermal pads that come with the xbox are placed under the board (there are 4 memory chips there) and should remain as they are. Don't remove them! They really help. Then I suggest to use 2 more under the GPU heatsink's memory chips.
No matter what x-clamp fix u did (drilling holes or not) the above must be done and avoid any bending of the board. Also drilling holes might code another problems but it is not proved yet. They say that in time because the bolts are on the air, don't provide support to the mainboard in the middle as they were supposed to (the original x-clamps). That puts lot of pressure when the system gets hot and some say the mainboard "cracks" under pressure, and you get RROD once more.
I find it logical so I use bolts (screws) that is exact the height I need (like original x-clamps) and I don't have to drill the case. So far I didn;t see any kit using my bolts, because maybe they are more expensive than the ones they sell
