Is it possible to tidy the internal wires/cables/ribbons out of the way??? It's surprising how a few wires can restrict airflow, usually from where it's needed most.
Depending on the type of PC case, it might need an extra fan or 2 adding to it. There's several styles available, standard square fans than mount directly to the case where the vent holes are, a 'slot-fan' that sits in an empty back-plane slot. These are handy for drawing heat away from warm graphics cards etc.
The big case fans (usually 80mm or 120mm), and slot-fans connect to a spare molex HD/CD/DVD power connector, there are smaller 'chipset' fans that usually plug into the mobo for variable fan control.
Ideally, the CPU heatsink should feel warm to the touch. If it makes you snatch your finger away, while shouting 'OUCH!!!' and other expletives, it's still waaaaay too hot
