If you're also looking at socket 462, there are a couple KT133A based models that come with (usually a single) ISA port and even support the Athlon XP family, including the Thoroughbred-B. Personally I don't think the regular Athlon or Palomino-based Athlon XP are worth using over the Tualatin, due to high power consumption. But a 1700+ T-bred B would be my ideal pick:
- it offers the best performance, better than both Tualatin-based Pentium 3 or Thunderbird-based Athlon
- isn't missing any instruction set support, since it supports MMX, 3DNow! and SSE
- power consumption is not as out of control as on Thunderbird or Palomino based Athlons. Still higher than Tualatin, but it also offers correspondingly higher performance
- the CPU itself is dirt cheap to find, unlike the higher clocked Tualatins. Motherboards that support one or the other should cost about the same though
A couple KT133A motherboards that support it (sometimes not all revisions) include Abit KT7A, Soltek SL-75 KAV and DFI AK75-EC. All are regular ATX size though; not sure if there are any models that are mATX and have both an ISA slot and support the Athlon XP CPUs.