VIA KT133 and KT133A based motherboards (old, I wouldn't consider them at ALL for a new system) take SDRAM. This is either 100MHz or 133MHz usually, the 133MHz performing better. There is also 150MHz and 166MHz SD RAM which supposedly increase overclocking potential.
VIA KT266 and KT266A based motherboards - KT266A are the best boards available just now - take DDR RAM. DDR RAM comes in three speeds usually. PC1600 runs at 200MHz, PC2100 at 266MHz and PC2700 at 333MHz. I would recommend PC2100 or PC2700 RAM. PC2700 is a good idea if you are considering to overclock the memory bus heavily.
The SiS 735 chipset takes both of these types of RAM
The above chipsets are for AMD motherboards, the KT266A is probably your best bet, with PC2100 or PC2700 RAM
For Intel motherboards:
The Intel i850 takes RDRAM, running at 400MHz (I think..?).
The Intel i845 takes SDRAM, running at 100 or 133MHz
There is also a chipset supporting DDR RAM, but I can't remember it..