hard drive dont effect too much as powerful graphic card or powerful CPU does, and as well, its heat will be outwards, not inwards. again telling, install without worries.
see my system, dissapating enormous heat, still installed a small fan at back, and that is sufficent. (but as well designed optimum air flow of the system)
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"