That's just the way it works; you'd have 3.4GHz of processing power available for each of 2 separate task streams. For instance, you could theoretically run a virus scan while at the same time browsing, downloading, and listening to music, your virus scan and your entertainment both being processed at 3.4GHz.
There is, however, a genuine speed advantage to multi-core processors; a machine mostly always is doing more than one thing at a time, the various simultaneously ongoing tasks essentially get broken up and the pieces are placed in que for processing. With multiple processors, you have multiple ques; less waiting time.
All that said, the fastest processor available is not of much benefit without adequate memory and a suitably responsive hard drive. If you really want the fastest machine around, get a fast processor, the maximum amount of the fastest memory the processor/MoBo/chipset/Operating System will support, and the fastest hard drive you can find. Going beyond that, the next bottleneck is video; get the meanest video card your resources will support. And of course high-end hardware-based (actual cards, as opposed to integrated onto the motherboard) audio, networking, USB, and FireWire solutions will boost overall system performance as well.
Its really very simple; just add money