Passive cooling for underside of GPU?

You typically don't need any additional passive cooling on the backside of a GPU. You could add some small heatsinks, but the change will likely be minimal. Something like these:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Keep in mind that GPUs and their cooling solutions are engineered to meet the needs of the vast majority of users. You should be fine without modifying your system.

Good luck!

Im just trying to figure out how to cool my card with 2 out of 3 fans. One of the fans in my card runs pretty loud, i suppose it vibrates or something. Gigabyte service is terrible. Last time I sent a card to them I waited for almost 2 months to get it back and it wasnt even fixed. I tried to find a fan like the one on the card but couldnt. So I thought that maybe there was a way to add a passive cooler instead of replacing the stock one. The loud fan is easy to unplug, the temps when it is are generally fine but Id feel safer with something replacing the unplugged fan.

You can try installing one of these (remove the stock fan assembly):

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Well yeah but like I said I dont really want to replace the stock, because removing it would mean my warranty becomes void.

Looks your options are fairly limited. If you case supports a side fan, you could blow cool air from the outside of the case onto the GPU. Adding passive heatsinks will do almost nothing.

I would not disconnect a fan. RMAing is your best long term solution (albeit painful).

I do have lots of space for additional fans I suppose. The bottom, top and half of the side of my case is pretty much mesh.

A side mounted fan blowing cool (outside) air directly onto the GPU will drop temps and fan speeds.

Should I also get a dust filter if its going to blow directly at the card?

That would be useful if you have a particularly dusty work area, not necessary though.

While you are actively answering=) If sound dampening mats are installed inside the case, without blocking any vents, how bad are they for the overall heat inside?

It really depends on how well you install them and whether they restrict any airflow. Keep in mind, if you have a "meshy" open system, the sound is exiting via all of the openings. The dampening material could help with sound but can case excessive heat issues.

What case (make/model) are you using?

Nothing too special. Aerocool pgs-vs-9

Not a bad case design. Adding the sound dampening material can help. You can also swap in quieter case fans (assume you are using stock fans).

Any ideas for the top and bottom of the case? Technically the open top means the heat can rise naturally, dont know if theres a point putting something up there to pull out air.

A top fan can also help. Just remember this, top and back fans suck hot air out of the case. Front and side fans pull cool air into the case.

As long as you have good front to back airflow, in most cases, you don't need the side and top fans. Adding those will definitely increase airflow, but could introduce more noise.

For multiple fans, you might want to consider a fan controller for better fan speed (noise) adjustment.

Alright time to shop. Thank you.

Have fun!