How much component temps will be effected by HDDs depends on how hot they run and how well the case fans in case are moving air through them .. as well as what other intake fans are supplying cool air to components. Link ciarlatano supplied has many posts about airflow and cooling. 5th post is good place to star. 11th post might help understand how fan specs work and how to choose fans that can maintain good airflow through grill, filter and past things like HDDs.
Phanteks Owners Club Ways to Better Cooling
i7 980 @ 3.55GHz =PH-TC14PE w/2x TY-143 fans =Crucial Ballistix 3x4GB =GA-X58A-UD5 =ASUS GTX580 DirectCU II =Enermax Modu84+ =Define R2 w/3x TY-140 case intake fans; all PWM controlled by CPU fan socketPhanteks Enthoo Primo MoBo Rampage III Extreme CPUi7 980X
@ 4.0GHz =R1 Ultimate
w/2x TY-143 fans =Dominator GT 6x2GB
=Rampage III Extreme
=ASUS GTX580 DirectCU II
=Enthoo Primo w/ custom castor base