Hi newcomer an welcome to the Tom's hardware forum.
The Antec900 is an ATX, m-ATX case and the Crosshair IV is an ATX mobo, so, u won't have any problem to install the mobo inside the case.
About the GPU, u can install a 5870 without problems.
thank you man, glad to know this
heres the specs and parts im getting for the comp if u wanna know :
case: Antec 900
mobo: Asus crosshair iv formula
processor: AMD phenom II x4 965 @3.4 (O.C @ 4)
Ram: DDR3 8gb ram (sorry dont rememebr the brand)
HDD: Wester Digital 1.5tb, 3.5" Caviar Green sata II HDD (and maybe a 160gb one for recordings
cooling: case fan's and Cosair h50 high performance cpu cooler
psu: Antec trupower 750w ATX ps4 with blue led
GPU: Sapphire HD 5850