I just installed a sapphire 4870 in my antec 900 case last night. It fit however I had to get rid of the second fan on the one hard drive cage that didnt have hard drives in it. This may not apply to your Mobo but I had to route the cables for the power button reset button and HDD LED underneath the back of the card because there was not way I could route it over top of it.
I too am interested in purchasing a Sapphire 4870 for my computer. The problem that I have, is that I have a relatively small case and before I open up my wallet I would like to know if it would fit. I have been searching for the actual dimensions of the card itself, but I cant find it. Does anyone out there know where I can find them or has anyone measured the actual card? If there is a topic about this already can one post the link to the topic? I am more interested in the length of the card, more then anything else.