As far as picking the pace up on your compiling, cpu and memory are
going to have more effect that a new graphics card. You can pick up a
used Pentium4 ~3.0GHz for less than $20, so getting rid of the Celeron
seems like a good idea.
It was the first processor with Hyperthreading (one physical core and
one virtual core). I would say it's relatively common, as it was a
top of the line at the time it was offered. When Hyperthreading is
enabled, you should see two processor traces in Task Manager (in
Windows). If you have Win98, then Win98 only supports one core,
so the second (virtual) core wouldn't be visible.