Dual PIIIs actually run Linux REALLY well. I used to have a Dual PIII/800 socket 370 board in my Linux machine (I gave it to my dad when I got the Mac Pro) with a GeForce 5700GT graphics card in it. That ran everything in Ubuntu 7.04 with style.
A lot of people take for granted the disadvantage of dual CPUs, especially in UNIX-like OSs. You don't just gain the power of another CPU, you get better overall performance in a heavily multithreaded environment like UNIX or Linux when the system is running a lot of single processes at once. That's one of the reasons OS X has prospered so well on Dual Core Intel chips where Windows XP, and to a lesser extent Vista do not do so well on the same rig.
A lot of people take for granted the disadvantage of dual CPUs, especially in UNIX-like OSs. You don't just gain the power of another CPU, you get better overall performance in a heavily multithreaded environment like UNIX or Linux when the system is running a lot of single processes at once. That's one of the reasons OS X has prospered so well on Dual Core Intel chips where Windows XP, and to a lesser extent Vista do not do so well on the same rig.