QUAD vs. DUAL value depends on what you're doing with the PC.
Actually for many applications it's not the HDD I/O that's limiting the speed,
it's the RAM speed.
If your applications do enough 'math' per megabyte of data they process then
the dual or quad core CPUs can be a great enabling value as long as they're
able to keep busy doing calculations on what's already loaded in RAM and
CPU cache, and the RAM's slower speed doesn't starve the CPU of data.
Typically quad core CPUs will be most effective at number crunching applications
where you're doing VERY complex or VERY repetitive calculations on data
where a significant chunk of it fits in CPU L2 cache, and the amount
capable of fitting in RAM is either the whole problem size, or is at least
enough to keep the CPU cores busy calculating at full speed for at least
10 seconds to more like 2-15 minutes.
It's also a benefit if you're doing heavy virtualization or running
server processes with dozens or hundreds of processes or threads
that keep busy doing high speed network traffic.
Typically this will be true of things like data compression, audio / video
compression encoding, image recognition, image filtering, video filtering,
speech recognition, engineering/scientific computational modeling, etc.
At ~ 2 GHz you're doing about 2 to 8 billion calculations per second per core,
so if you can use more than 4 billon calculations per second
on a problem that can be calculated in pieces, quad core may help.
If you're not doing calculations that max out your CPU core(s) for
at least an hour at a time per day, then increasing your CPU cores
won't save you that much time.
If you can do 8-30 billion calculations per second with a quad core CPU and
yet you need less than 3GBy (write) to 6GBy (read) per second of
main RAM memory access then you're able to use quad core without being limited
by your RAM speed. Otherwise, you need more PCs running in parallel since
you're limited by your RAM speed and can't feed the CPU enough data from RAM
to keep your cores busy.
If you can calculate for well over 2 minutes with the amount of data that fits in
your RAM before you need to load / save it all again from RAM
(more like 15 minutes if you have a lot of RAM in use), then you're probably not
limited by your hard disc speed even for slow discs. Otherwise, you're probably
doing hard disc I/O of more than 5-10 MBy/second on a sustained basis, and
your drives may not be able to transfer data to/from RAM fast enough to keep
the RAM and CPU fed fast enough; use a RAID or something to get more I/O
to your RAM and CPU or you're wasting money on adding more CPU cores.
For something like Folding@HOME, SETI@HOME, etc. you could have as many
CPU cores and as much RAM as you could possibly buy and still it'd be kept fully
busy 24x7x365 calculating even with the amount of data you could download/upload
on a good broadband internet connection.
Encoding a ~ 30 minute high definition video to DIVX, for instance, probably
takes something close to 1 hour on a moderately fast single core CPU; it's the
kind of thing that'd run literally 4x as fast on a quad core CPU.