What exactly are "North and South bridges?"

[The_Absolute]

I was hoping to upgrade my motherboard this Christmas. I know I need an LGA 775 socket, and I also know that even though a motherboard may have the same socket type as your CPU requires, the bridge types may be different, making it completely incompatible with your processor. Right now, I have a C2Q Q9400 (2.66 GHz) I'm not sure about the bridge types, so I don't know which LGA 775 motherboard I should buy.

[MATLABdude]

If you're looking for what they are, here's a primer (they handle the interfacing from the CPU to most other parts of the computer):
http://en.wikipedia.org/wiki/Northbridge_%28computing%29

You're probably thinking of a bridge set (more commonly referred to as a chip set, with both north and south bridges). The chip set must be compatible with your desired processor, but happily, most vendors are more than happy to tell you which motherboards work with which processors (and vice versa).

If this is your selection criteria in the first place (i.e. figure out a chipset and then narrow down the number of motherboards you have to research), you can look here:
http://en.wikipedia.org/wiki/List_of_Intel_chipsets#Core_2_Chipsets

NVIDIA and VIA also make compatible chipsets (some with more bells and whistles than others):
http://en.wikipedia.org/wiki/Comparison_of_Nvidia_chipsets
http://en.wikipedia.org/wiki/VIA_chipsets#AMD_.2F_Intel_processors

But it's probably easier to go to, say, TigerDirect or NCIX.com, and start searching for motherboards with a certain socket (e.g. LGA775), whittle down by manufacturer / features, and then see if the ones you click on are compatible with the Core2 Quad.

[bitrex]

One thing to keep in mind is, even if the chipset and socket type are identical on the new motherboard, the BIOS revision may not support certain processors. This is usually less of a problem in your case when you're using a new motherboard with an older processor, but it's definitely a problem when using a new processor with an older motherboard. One (personal) example: AMD was nice enough to make their newer AM2+ processors backwards computable with the AM2 socket, however Asus doesn't have a BIOS update available for older motherboards to allow them to use the newer processors. There's absolutely no physical reason that the older motherboard can't fully support the newer processor, but certain features are disabled when you use the current BIOS.

In contacting Asus about this they've plainly stated that they don't intend to release any new BIOSes for their aging equipment - I suppose it makes business sense in that they don't want to expend a lot of effort just to undercut their new product sales, but as a consumer it's galling and certainly helps defeat any plans AMD might have had to encourage users to update their system on a smaller budget.

End rant!