if your just keeping the stock cooler then i would deff get some extra fans even if your not overclocking, the cm doesn't allow you to do that
300 has better stock airflow, allows you to add 3 more 120mm fans, 2 on front and 1 on side pannel
cm 5 only allows you to ad a 80mm to the side, and if u get a aftermarket cooler chances are you wont be able to add it on there because its directly over the cpu.
im just saying spend $5 extra dollars and get a way better case
if u want to stay cm u might look at the 590 also:
Newegg.com - COOLER MASTER Centurion 590 RC-590-KKN1-GP Black SECC / ABS ATX Mid Tower Computer Case - Computer Cases
better case then the 300 imo, better cooling, and awesome for micromanagement. has the ability to get a side window:
Newegg.com - COOLER MASTER RA-690-KWN2-GP CM 690 clear side panel with one 120 mm blue LED fan - Case Accessories