These would most likely be the ambient temps in the room the rig will be located in.
Not for long
You'll find out quickly that these rigs make excellent space heaters. I plan to duct mine into my furnace this winter to warm the whole house.6870 is overpriced for mining, +$50 USD over the 5830 which has the same number of shaders.
Go for the 5830, or go for the gusto and reach into the 6950/70/90 area.
You'll probably want to do some experimentation on case vs caseless; if you're going to have a harddrive or other moving components (other than fans, those should run fine), you should probably use a standard PC case.
If you can boot from USB or the network instead, go caseless - with ambient temps like those you'll get some pretty nice wintertime overclocks