OCing does increase the heat of your NB(SB not as much, it's hot but normally stays that way, thats why the blocks are for NBs only), many times by alot, especially if you increase the voltage.
in order to overclock your CPU your motherboard has to be able to handle the speeds too, the more you overclock the more your making your motherboard operate at above factory standards, the heatsinks on the chipsets installed are only to handle factory settings and only a bit more, in order to OC your CPU you gotta go through your board and OC that hand in hand, and OCing anything in computers adds excess heat.
if your thinking about watercooling, you should only do it if your OCing by a large margin. if not, a fan or a large fan less heatsink should be find. If you OC and you want to air cool swiftech has a great fan + heatsink combo for chipsets, heard it works wanders.
the DS3 being passively cooled does need airflow across it, especially if you are running 400+ fsb. the stock cooler or any blow-down type is more than enough. If you are using one of those side blowing types (ultra-120, tuniq tower, whatever) you'll probably need to add a fan.