software does not have to be written for dual core in order for you to leverage the benefit... for example if you are rendering a video file and at the same time decide to use your system for something else which needs a little resources it will not be impacted and things continue smoothly.
A boring, but obvious, example would be if you ran the folding client. With a dual core you can simply run two and immediately double your throughput.
Really it depends an awful lot on exactly what you do with your system and how much value you set on bangs per buck - the sort of specs LeeJend mentions carry a premium price tag ATM, but on the other hand entering the dual core arena with a 4200x2 has now become pretty cost effective.
Now if you were talking about 64 bit software - well I'd say that was another story- there apps and drivers matter, but not everywhere is going to need the lofty headroom 64 bits offers so the apps using it (as to being simply compiled within it or using the emulation of the OS) will be few and far between.
Hope this makes sense and is useful to you. If you had more specific need of information you need to refine your question or give us more background information I think.