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.