This difference is that the original Athlon has an off-die L2 cache that runs at a fraction of the CPU speed...fraction varies by the how fast the CPU is...the faster the CPU, the smaller the fraction. The original Athlon also uses a <b>slot</b>-A form factor.
The Thunderbird Athlon has an on-die L2 cache that runs at CPU speed and thus is faster (I don't know how much). Most, but not all, Thunderbirds use a <b>socket</b>-A form factor, which all the new motherboards use.
So if you're starting from scratch, get a socket-A Thunderbird and a Socket-A motherboard.
Mike