customize casing

I want to customize my build and setup, but don't really know what to do, buy and what i want. Please help. Thx.

So as the title says, i have a GTX 560 ti I bought it 2nd hand from my friend, when the gpu is idle no apps open it's around 38-42 Celsius, but when i play the game nba 2k16 in all high settings, it shows 85-90 Celsius temp. Fan Duty is always @ 40%. I have 3 cpu fan, 1 is big facing the...