What new K6 processor can I put in a older motherboard?

AMD gave the NexGen design team their own building, left them alone, and gave them time and money to rework the Nx686. The result was branded the K6 processor, introduced in 1997. The redesign included a feedback dynamic instruction reordering mechanism, MMX instructions, and added the missing floating point unit (FPU). It was also made pin-compatible with Intel's Pentium, enabling it to be used in the widely available "Socket 7"-based motherboards. Like the Nx686 and Nx586 before it, the K6 translated the Pentium compatible x86 instruction set to RISC-like micro-instructions. In the following year, AMD released the K6-2 which added a set of floating point multimedia instructions called 3DNow!, preceding Intel's SSE instructions, as well as a new socket standard called "Super Socket 7", that extended the front side bus frequency from 66 to 100 MHz.