There are lots of differences to these chipsets: which processors, processor interfaces, memory types and quantities supported, bus speeds, hardware monitoring, and price.
The best way to get familiar with the differences is to read the motherboard reviews and make a table of chipsets and features. You'll learn about motherboards at the same time, which after all is what you'll buy, and also learn which motherboard features are really features of the chipset. I've done this and my table is pretty big now.
Then read the motherboard forums and newsgroups for a while. You'll start to get a sense of which chipsets are causing a lot of compatibility and setup problems. In particular the VIA chipsets seem to be more prone to this, at least by a simple count of problem reports in the forums.
A few short answers: the i820 uses RDRAM, so if you're using SDRAM it's not for you. The VIA KT133 supports Socket-A (AMD) processors, so again it's not for you. But the VIA Apollo Pro 133A, i440BX, and i815 all support Pentium processors (various interfaces) and SDRAM. So have a look at motherboards based on these. The BX is an older chipset, on its way out now, which of course makes it less expensive. It officially only supports a 100 MHz FSB, but see the AnandTech article on overclocking the BX.
Good luck,
Andrew.
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