Ok, all of the memory you'll ready about is support by AMD, except for PC800, PC700, PC600 which is bad RDRAM and only used by Pentium 4. Here's a description om the Athlon mem types:
PC100: Runs at 100MHz, isn't popular anymore, lowest performance, and has given away to PC133
PC133: Runs at 133MHz, most common RAM, low latency
PC1600: Run at a 100MHz DDR bus, meaning that it really runs at double it's clock thus getting the effective rate of 200MHz, isn't a good option, PC133 most of the time performs better with Athlon
PC2100: Runs at 133MHz DDR (266MHz effective), Is the highest performing type of RAM on Athlon, costs the same as PC133, you should get it.
For a mobo that supports PC2100, either Epox 8k7a, or MSI's K7 Master are great DDr boards, for PC133 Iwill's KK266-R, and Asus' A7V133 are also great mobo's.