What will you use it for? The second CPU keeps the system responsive when one CPU is completely consumed with an ongoing task. In this situation it can be a night-and-day difference. If you have multithreaded software such as mid-to-high-end 3D animation/rendering software, there's another place where you stand to benefit, either by having both CPUs render at once to shorten the task, or by having a "spare" CPU to use for other things while rendering's going on in the background on one CPU.
Aside from those situations, a fast single-CPU system is preferable to me. I replaced my dual-733MHz P3 system with a single-CPU AthlonXP 1700+ (exact same total MHz) and it is much more responsive for general day-to-day usage. On those occasions when I do 3D rendering for hours on end, a modern dualie would be nice, however.
Some people swear that dualies are "snappier" even when doing ordinary day-to-day stuff. Having had three myself, I've found that they're a specialty tool, as I mentioned above. Great for what they're good at, but they lie dormant the rest of the time. But it's your money. 🙂 Do consider other ways to spend it, such as 15000rpm SCSI hard drives.