It matters marginally. IIRC Leadtek 6600GT cards have good ones. EVGA ones are based on the reference NVidea heatsinks, which are always sufficient.
Overclocking is raising the speed of components beyond their specifications. Similar to using nitrous oxide in cars.
It's not often too useful for video cards, as they usually run pretty close to their max spec anyway, so for much overclocking, you either have to make very careful purchases, or spend more money on increased cooling and voltmodding.
I can see you having problems with a 300 watt power supply if it isn't a good quality one...
Overclocking is raising the speed of components beyond their specifications. Similar to using nitrous oxide in cars.

It's not often too useful for video cards, as they usually run pretty close to their max spec anyway, so for much overclocking, you either have to make very careful purchases, or spend more money on increased cooling and voltmodding.
I can see you having problems with a 300 watt power supply if it isn't a good quality one...