Cooling options for my computer

I download a lot.Since past few months I've been facing a lot of software problems and crashes.I am sure this is due to the overheating of the motherboard, graphic card etc.I have an Intel 945 chipset motherboard with a GeForce 6200 LE graphics card.
Now to get to the point, I have no idea about any cooling options and how to go about them.I think I would need a couple of fans.Would that do?Or do I need the thermal paste too(dunno how to use them)?What are heatsinks?

I would require your advice on them and how to go about installing or using them.I am ready to spend close to 3k for this.

Heats while Downloading? Just a couple of Intake and exhaust fans would do.
Dont forget to clean the dust in the CPU heatsink.

Total expenditure: Rs. 100/150 per fan (80mm/120mm)

Installation:
Fan Screws come included with cabinet. if not just buy a few along the fans.
Open cabinet, place the fan at the rear exhaust while aligning the four holes of the fan with that in the cabinet.
Insert the screws.
Connect the fan's wire to a white/black colored 4pin connector which exists on the motherboard.
Repeat process for front fan.

Start the system.
(Its not that tough!)