I have a dual Intel 233MMX system and I'm not seeing much performance increase due to memory bandwidth, software, and other limitations not relating to the CPUs. It feels cool to own a dual system, but the price/performance is not very good.
I'm gonna built a 1.2GHz Thunderbird system and I bet it'll beat the crap out of my dual 233MMX 😀
If you have the extra money to spend and have SMP aware software then go ahead, otherwise get a higher clocked system.