A K-6? Specifically 6? I believe (correct me if I'm wrong) that the K-6 had a serious problem with its FPU, causing (among other things) seriously slow gameplay on Bleem! (and probably others too). (i.e., a p1 166 MMX could play faster). This was fixed with the K7 or above processor, which put it on even (perhaps surpassing) footing with Intel. Hope that helps, and PLEASE correct me if I'm wrong.
Killershots is right, the k6,k6-2,and k6-3 have weak FPU performance, however they DO have a better Interger Unit so business apps perform better. Unfortunately, 3d game performance is severly lacking (but not as bad as KS was saying). Though a P3-500 is far superior to a K6...the k6 is about as powerfull as a p2-350 or something
Actually, k6, k6-2 and k6-3 have a good 3d performance if the software was made to use its 3d!Now instructions. As not many softwares does, its FPU performance is usually lower than a PII. However, AMD has surpassed Intel now that Athlon has a better FPU and can use an upgraded version of 3d!Now(also called 3dnow). PII can only use MMX that does not accelerate FPU instructions. PIII can use SSE that is Intel's answer to 3d!Now.
Anyway, both K6 and PII are old generation processors and that time intel was still in its wonder years. Nowadays, if you want a CPU and you are in doubt between an Athlon, a PIII and a PIV, amd's one is surely the best choice.