Motherboard Specifications

Hello,

I am interested in bying a PC. I used to have an IBM PC Pentium II 450MHz and I tried a Pentium III 733MHZ and it seemed slower than the older one. I am wondering if there is a problem with the motherboard. Does the motherboard play a major role in the speed of a PC? Why are there so many different prices for motherboard? Are there certain things that I should pay attention motherboard wise before bying a PC?

Thank you in advance,

Panayiota