I'm new to building my own computers and I was wondering about maximum temperatures.

I understand that the room temp can affect the computers temp but how relevent is it? I understand that if my room temp is 20°C none of my computer parts could be lower than that. But if my room is 27°C instead of 20°C would this cause my computers parts to heat up more/faster?

My new computer I built myself for gaming is

i7 2600k
16gb ram ddr3 1600
hd6970 2 gb
240gb ssd ( bought a nas with 3 2tb drives in raid 5 for my home network )
850w modular psu

I also have my old hp computer

i3 2120
8gb ram
hd6770
1tb hdd

I also have 3 laptops in my household, but I am not worried about their temps, they heat up my legs but they are never under stress.

Due to size and money reasons I used an old case and it only has one of the sides left on it. Is this bad for the computer and will the extra dust cause problems? Or should I leave it this way or take the missus wrath and buy a case? If so is there any certain case I should get? I don't care about looks I just want card reader and usb slots and for it to run as cool or cooler than now, my case has 1 fan.

Also what are the max temps for my new and old computer parts? Is 40°C under load ok for my CPU, what about 70°C for my GPU is that ok too, or should I worry? What are normal and safe temps for my components?

I have looked around but there seem to be lots of different answers. I know that 100°C is bad but I want my parts to last as long as possible and this site always seems to give good replies without arguing or flaming.