I don't have any experience with motherboards using the SiS 741GX chipset.
I have lots of experience with some motherboards using the VIA KM266 chipset. AFAIK the "Pro" version just adds support for 333MHz FSB. I have some experience with the VIA KM400 chipset. AFAIK the "A" version adds 400MHz FSB support.
I'd say forget about the KM266 and KM266 Pro chipsets. The KM400A is okay if not much more, but "official" 400MHz FSB support may be nice if you're going for a high-end small system. KM400 chipset boards can be had for around $50. I've always had good luck with Biostar, DFI, Shuttle boards using these chipsets. I have NOT had good luck with PCChips boards using these chipsets (multiple boards had strange BIOS problems, will boot to "checksum error" now and then, totally unacceptable). I've had mixed results with MSI boards using these chipsets. The ones that worked, worked, but too many RMAs. The Shuttle boards sometimes are Flex-ATX, so they are smaller and sometimes don't bolt in properly in some cases. The DFI boards give SATA at a low price point.
For a few $$$ more you can get a Biostar M7NCG-400.
What are you going to use this for?