Which CPU

I am looking to buy a new PC. I hate my PCs becoming antiques every 3 years. I am trying to understand which right combination gives me best chance to upgrade easily in the future. I understand issues like bus speed, chipset, etc will affect my decision. I understand the 533/400 MHz bus speed. I understand the .13 micron CPUs. I read about the chipsets. I don't quite understand them. That is why I came here for help from you experts. I want to buy a PC soon that I want to be able to upgrade, if needed, with a newer CPU couple of years down the road. I am not looking to buy the fastest CPU, but the right combination of bus speed, chipset, etc. So, I am not looking to buy 3.06 GHz CPU.

My question is simple: What combination of CPU speed, bus spped, Chipset would you folks get now and hope that you would be able to upgrade if needed 2 years down the road, without having to buy a new PC. Thanks.