The southbridge can just be cooled with a simple heatsink for it is not that warm compare to the northbridge. The northbridge however can be very warm like the Nforce4, so it would definitely needed to be cool with either a heatsink or heatsink fan. As for my northbridge(Nforce4) on my motherboard(Gigabyte GA-8N SLi Royal) is becomes hot to the touch, the heatsink/fan anyways. On top of that the bearing got bad and makes annoying whizzling sound. So I included it with my water cooling loop along with the cpu. Got the cpu temp up to 2 degrees celcuis but no biggie, the good thing is now there's no annoying sound and not hot anymore. I would have gotten an after-market north bridge heatsink/fan but it's too close to the upper graphics card.
The northbridge includes the memory controller and handles communications with the PCI and PCIe slots and the Lvl2 cache (although, AMD chips have an on die memory controller). Since it is a critical piece of hardware you can never be "too" careful with it. The southbridge pretty much just communicates with the various ports on the motherboard (sata, pata, usb, firewire) it will get warm but usually doesn't need anything but a heatsink because not as much traffic flows through it..