The key here is not that much the CPU but the available ram.
We have experienced very good result with a Pentium 133 driving
4-6 X terminals. Depending on the programs used, memory between
96megs to 128megs is required.
Note that again, faster CPU and more RAM is always better
but a lowly P133 with enough ram is a good performer. Most
people have experienced with P133 and 16 megs. Throw some
Desktop users (office tasks) work by chunk. They load a file
and then read and then type a little and then preview. With
4-6 users, this means that they will seldom use the server
CPU at the same time. This is why they will get in general
excellent performance. Even if they do, you will find that it
does not show that much, thanks to Linux efficient multi-tasking.