heres a compare link between the 3 compare link here^^ and generaly they are pretty much the same. the MSI has slighly higher core clock because its factory OC'd and the iceQ runs abit cooler because...well its cooling is frikken huge.
i would go with the MSI just because i think the diamond card is biosed locked which is a bitch if you planned to OC it.
also as yelowcard said you can move around the hdd bays to probably make it fit. but in my opinion i dont like having the graphics card take up the entire case messing with cable routing and airflow, also I found the iceQ to be way to heavy for my liking and you apparently need supports when transporting the card(which ofcourse you dont get with the package)
also you might wana look out for the new Tahiti LE cards like this one, those are beast
Yes, the bios is locked but I dont plan on OCing. The Diamond is $20 and $30 cheaper than the other two and that is before any rebates. Gaming wise how far apart are the three of these cards? If there is only a few frames difference in games then why shouldn't I go with the Diamond?