Faster FSB for a given chip manufacturer will provide overall performance improvement unless the slower chip has better memory management (Intel vs AMD debate).
Dual core will provide much faster multitasking (running 2 programs at once, like video/audio editing on one core and regular desktop tasks on the second). But until multithreading application programs are available single tasks will mostly still run at the same speed as if you had a single core. A third or 4th program may not run any faster unless some of the programs are coded for multithreading.
If you want a performance system at this point it is probably best to get a 64 BIT dual core CPU with at least an 800 mHz FSB (preferably a 1066 FSB) that supports DDR2 RAM with speed ratings up to 800 Mhz.
AMD AM1 socket to support this configuration is due out soon. Intel already has this on the street but they are doing a major CPU revision latter this year that I think uses a different MB/CPU Socket. i.e. wait a little while if you want a solid upgrade path for latter. You can buy now if max performance and upgradability is not that important. The DDR2 800 is not availabel yet either, only DDR2 667 which is not any better than old DDR.
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.
With dual core, you can supposedly encode a video file while burning a DVD at the same time. Each process uses a separate core so that neither process interferes with the other. My laptop is Intel Core Duo, but I haven't really pushed both cores at once so far.
I'd actually be slightly surprised to see games utilize it (they already support it) - remember they aim to code for the lowest common denominator (though they'll push specs on GFX etc to put on a show) but really aiming to maximize market share - so exploitation would slightly depend on gamers ALL going dual core - and a hard core gamer will care purely about single core speed, so would be wasting money for a dual core....