All Athlons and Durons are k7 CPUs, including the Athlon XP. The k6 series are called (drumroll, please) - K6, K6-2 and K6-3. Some other variants may exist. Using k7 as the CPU type in kernel configuration will be the best option for you if you have an Athlon XP.
There are actually 32bit k8 CPUs as well but they are not very common. I have one in my HP nx9105 laptop (which is usually sold as the Compaq R3???z, with 64bit CPUs).