SiS chipsets are inexpensive, sure, but they've been outperforming RDRAM based Intel mobos for a while now. Don't believe me? Check out a review on Tom's Hardware Guide of the SiS 648 chipset. And keep in mind that Intel wouldn't validate faster RAM for it's DDR based chipsets because its performance came too close to its flagship product while being signifigantly cheaper.
http://www.tomshardware.com/mainboard/0 ... index.html
The SiS645 performed very well as well.
http://www.tomshardware.com/mainboard/0 ... index.html
It seems, though, that your PC store was mistaken, unless the BIOS in fact lets you run a 533mhz FSB, but I wouldn't risk it if the mobo maker hasn't validated it. Go for another chipset like the SiS648. Remember, though, that the 650 also has integrated graphics and the Asustek motherboard has no AGP slot for upgrades.