Hey guys, I am new here, and I have been having the same problem as gostev88 with my similar system.
My Asus M4A78-E northbridge (790GX) used to run nice and cool at about 42-45C at idle. Now it runs at 52C at idle, and I was concerned with the drastic temp increase. I felt the northbridge heatsink, and it was hot enough to make me pull my hand away from it quickly.
TC91 has an excellent point with saying to increase the fan speed on the 4890 reference graphics card. Upon manually increasing the fan speed to about 40%, the northbridge temperture went back down to 42C. I guess that since the 4890's gpu runs at a warm 60C at idle (not overclocked), it appears that the heat is rising up into the northbridge heatsink. I think Asus and other motherboard companies should try to invest in at least some fan assisted cooling of the northbridge, especially when they promote O.C.
I also agree with Sylvanas that the thermal compound that comes on the motherboard from the factory is not very good. I took the liberty of ordering a Thermaltake Extreme Spirit II chipset cooler because it seems like that will help a whole lot in bringing the temps back down to normal. The cooler has got pretty good reviews, and wont set you back too much in price.
I will let you guys know how well it does when I get it installed, should be on Thursday.
Thanks to everyone for the extremely helpful info.