as long as you have a decent amount of air going through your case, then the large passive heatsink that's on your board would be just fine. i wouldn't bother replacing it with a tiny fan that's going to add that little bit of extra noise and has a chance of breaking down in a year or two. not to mention, that little thermaltake heatsink has practically no surface area on it, so it can't be very effective.
but if you don't have any more than one case fan, then the active cooling might be a good idea, if you overclock.
if i wanted to add active northbridge cooling to that board, i would buy my own 30 or 40mm fan and find a way to attach it to that large heatsink that's already on it. now that would cool it down. definitely be cheaper, too.
the northbridge has the large grey heatsink on it, next to the cpu. your southbridge is the large black chip next to your agp/pci slots that says "MCP-T" on it. the southbridge doesn't really need to be cooled.
the northbridge does though, hence it has the heatsink on it. like i said, that heatsink should be plenty to keep it cool. but if you want to overclock or just don't have much airflow going through your case, then active cooling might be a good idea.
it's supposed to be sorta hot. after all, it's dissipating heat.
as far as i know, northbridges are quite tolerant of high temperatures. some nforce2 boards are shipped with very short passive heatsinks on them, so i'm sure they get even hotter than yours and still run fine.
ok compared to a Opteron K8 HSF which is 35dB; the tiger is 23dB which is substantially quieter. this is because the fan is not trying to shift so much heat - that would be produced compared to a CPU. so it doesnt need to be higher RPM and most likely other fans will drown it out