in computers cpus, there are 2 ways to make things work fasters (in general anyway);
either make the cpu work faster - speed, or make it do many things at the same time - hyper threading in the past (maybe ur old pentium 4 has this already), and multiple cores now days.
but the hardware advances are faster then the software development. if you use ur computer to do one thing at a time (play one game, surf the net or using word), multiple cores won't help you much. but if u tend to either work with many software at once, or use special software that can make use of two or more cpus at one time, then you will defenitly feel the change.
but this is not all - you should always take into account the price you pay for this upgrade. in fact, it might even be better to spend money on some more ram then to pay extra for a dual core cpu. but again, u have to take into account what u do with ur pc.
i suggest you read about the subject for a bit, and use charts like the one from tom's hardware to make the right choice;
you of course don't need to read everything, but at least make sure you understand the differences between the options and read the conclusions the writer get to.
http://compreviews.about.com/od/cpus/a/dualcore.htm
http://www.tomshardware.com/2005/04/05/the_pentium_d/
http://www23.tomshardware.com/cpu.html
http://www.tomshardware.com/2006/07/18/would_you_buy_core_2_duo_today/
http://hardware.slashdot.org/article.pl?sid=06/07/17/0612202