the chipset is what every motherboard is based on. it's comprised of a northbridge and southbridge and basically is the foundation for a mpotherboard. a chipset is devised by a company (Via, SiS, Intel, AMD, Nvidia) and manufacturers (asus, epox, abit, etc) produce boards based on the chipset. the manufacturer can add or modify chipset features to a certain extent (putting in a different sound proccesor, adding on raid, etc).
so choosing a board based on it's chipset is way more important than choosing based solely on brand.
Via makes...
KT series (kt133a, kt266a, kt333, kt400, etc) among others. the KT series are their performance-based AMD chipsets. the KT400 is the newest and a very decent chipset. they also make good intel chipsets.
SiS makes...
not sure... but most SiS chipsets are budget-oriented and not the best for power-users. though i heard they make decent chipsets for intels.
Intel makes...
chipsets for their processors, of course. i think these are probably the best if you're going for an intel system.
AMD makes...
likewise, chipsets for AMD processors. but not the fastest or most feature-rich of chipsets. usually cheaper.
Nvidia makes...
nforce and nforce2.
the nforce2 chipset has kick-ass onboard sound that rivals that of the audigy2 and other high-end cards. it also has dual-ddr which, when used with 2 equal sticks of memory, can double your memory bandwidth. so 2x pc3200 = pc6400! and some versions of the nforce2 chipset have onboard video capabilities equal to a gf4 mx.
i recently made a chart laying out the features of several different nforce2 boards from diff manufacturers. i attached it for you to use. the prices are a couple weeks old so do your own pricing.