Instead of a pci gpu, why not an evga uv+? It might be a better solution to your problem as it is external. No worrys about clearance or impeding airflow. They work really well for basic desktop and video playback upto 720p, but not for gaming.
Where can I get a evga uv+ from? and are you sure it does not support 1080p? http://www.evga.com/products/pdf/100-U2-UV19-TR.pdf since that one says it supports high resolutions. But they seem to be only available in USA... which sucks. I'm from England.
Well I've only had personal experience with the model 12, which isn't as good as the 19. A quick google tells me pctitan and yoyotech supposedly have them in stock.
EVGA USB VGA Adapter are you sure they actually work in Sli though? Cause the nvidia drivers/cards themselves stop me from using more than 2 monitors. Wondering if this would just waste more money and not work =/
Yeah it would work fine, it's basically the same as adding a third card, except through USB. You can leave SLi enabled as far as I'm aware as the monitor won't be running on the SLid cards