Just take the cooler off and get it done that way as the screws for the plastic cover can only be accessed that way as the heads are on the other side.

Just look at the back of the card :
You unscrew things you have to (ie. screws that connect to heatsink), and after you unmount it from card, there should be a second set on it's underside.
You can't clean this without removing cooler itself, so thermal paste will be required (since old one will be dry as desert and probably hard as a rock after all these years*).
*I'm assuming, no one ever did that before.

Also, dismantling cooler itself (to get to dust), may require a REALLY small screwdrivers.

PS. Reference 6800 cooler can be cleaned without removing it from graphics card.

agent_x007 wrote:

Just look at the back of the card :http://www.vgamuseum.info/images/palcal/nvidia/518_nx6800gt-t2256e_bottom_hq.jpg You unscrew […]
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Just look at the back of the card :
You unscrew things you have to (ie. screws that connect to heatsink), and after you unmount it from card, there should be a second set on it's underside.
You can't clean this without removing cooler itself, so thermal paste will be required (since old one will be dry as desert and probably hard as a rock after all these years*).
*I'm assuming, no one ever did that before.

Also, dismantling cooler itself (to get to dust), may require a REALLY small screwdrivers.

PS. Reference 6800 cooler can be cleaned without removing it from graphics card.

Jewler's screw driver/eyeglass repair kit works great. Grab a 25g tube of Arctic Ceramique for $7, just ran dry after 4 years! And its not conductive.