if your looking to play games get 2 7200rpm drives and raid 0 em. either that or get the ssd. reason being is your current drive is to slow to enable high transfer speeds required to stream game data and stop texture pop.
If you have the money get it all.
If not, do the hard drives first, they'll give the system the biggest boost overall.
The mouse when you become unhappy with what you have.
The heatsink, case and fans can wait.
Got to ask, is that 2.5" drive a 7,200rpm OR 5,400rpm, the 5,200rpm hard drive is not one I'm familiar with.