Get a dual core, the Pentium 4 is still fine but a dual core will make a massive difference.
What's good in a CPU: more cache helps, more GHz or clock speed helps, a higher FSB helps if you have the motherboard to back it up, and dual or quadcore is also good.
Basically, if you're buying a new processor now, look for Intel Core 2 Duo or Core 2 Quad, or the Pentium Dual Core series.