The Athlon Thunderbird series was a killer processor! I had a 1.4Ghz. It got very hot, but man it would scream!
As for that "PCI looking slot", that's called Slot1. Some early athlons also used this slot(well, SlotA in the case of AMD) This was before they could fit the cache on the processor. Before the slot processors, I beleive most Cache was located on the motherboard.