I have the same board and I'm running the bus at 334Mhz. The northbridge heat sink was hot to touch. I just attached the 40mm fan directly to the heat sink using the screws provided with the fan. Now the heat sink is just warm.
56c is a bit on the high end, but if your not having stability problems its probably nothing to worry about. If your temps start getting into the 60 or 70s then its time to worry. If I remember correctly that motherboard has a passive chip set cooling solution? These do end to run a bit hotter then active chip set, you give up a bit of cooling performance but gain a more quiet system. Passive coolers rely on the airflow inside the case more so then an active cooler. What kinda case are you using, how many case fans?
OK, first thing to do is to make sure your bios settings are correct and your not accidentally overvolting or over clocking the north bridge. Then you wanna make sure all your wiring is as neat as possible to allow good airflow, also make sure your case fans aren't set to a very slow speed, remember your heatsink is relying on your case fans to cool it. If that doesn't help next thing to do is to check and make sure the heat sink is making good contact with the chip and has the correct amount of thermal compound applied. So you wanna pull the heatsink and clean it and the north bridge chip, next check to see if the heat sink is flat and not concave/convex. If it is you'll have to go buy some sandpaper 200, 400 and 800grit and lap the heatsink till its flat. Then get yourself some good quality thermal compound like Arctic Silver or similar and apply according to the recommendations on the tube, then re-install the heatsink.
If at this point your temps are still high your going to have to either live with he temps, if your not having any problems with stability thats probably your best bet. Or you can purchase a high performance chipset cooler, Thermalright and Swiftech both make good ones but you'll have to make sure they will fit on your board.
I'm running an older AMD 4200x2 (S939) on a DFI NF4 UltraD motherboard and I also have pretty high temps of around 50-55c under load. Ive had these temps since the day I built the system two years ago and it runs perfectly stable even with a 600mhz overclock. Now I know you cant really compare our systems but I don't think you should worry to much about your temps its not that high really. Also I forgot to ask, where are you getting that temperature reading from?? Ive seen different temperature monitoring programs vary by as much as 10degrees compared to each other. So you may wanna try a few different programs as well as the bios and see what kind of readings you get. Well good luck hope this helped.
Yes, the motherboard has a passive cooler. My case is a Thermaltake Soprano DX, it's got one intake, and one exhaust fan. The airflow isn't brilliant, but it's not bad either. The MCH and FSB voltages are at "normal" in the BIOS. The NB temp doesn't seem to change even if I add 0.1 volts to it.
The heatsink can be swivelled a bit, but I think that's normal? Supprisingly, the NB temperature does not seem to change even with the case open and with a small fan blowing on it.
I think the system is slightly unstable. It passes 14 hours of Memtest ok, but it fails the Orthos CPU+RAM test. Could an overheating northbridge be the cause of this? I'm pretty sure the CPU is stable, as it passed 12+ hours of the Orthos CPU tester when I overclocked it a month or so ago.
I think I'll reseat the NB heatsink and apply some AS5 compound when I get some spare time. Pretty annoying how the motherboard needs to be removed to do this.