1. Leave the SB alone, it doesn't even get hot
2. Many coolers fit the NB, personally I am using an old Alphacool SNB-A I had lying around - cooling the NB with water is a good idea, since it needs to stay cool for high OCs
3. If you are running a highly OCed Quadcore (and only then) you might consider cooling the PWMs with water, too. The stock PWM sink is exceptionally good, and only needs improvement for 200W+ CPUs (like mine -.- )
PWM blocks are made by mips, as well as a matching NB block (a little expensive though)

Here is my LT X38 (layout identical to the X48):



Ignore the CPU cooler, my phase unit was down when the picture was taken...