Do i really need a quad core?

When new to the DAW world, it is very easy to start spending money because you think you need this, that or t'other piece of equipment/gear/software plugin, etc., etc., etc.  Did that myself.

It is far better to purchase things when you actually have a need for them.  Otherwise you risk end up buying stuff that you never actually use, and end up not having the money for something it turns out that you actually need.  Far better to learn how to use what one actually has than buy a bunch of things and not really know how any of them work.

If the dual processor is easily doing what you need, then there is no real reason to replace it.  If your projects were to start to regularly hit say 80% of your CPU's power, then one probably should start considering an eventual upgrade.

Where one's situation potentially changes is when one is running a wide variety of plugins, software or an other item that requires a great deal of processing power.

In general, when recording a small number of tracks, not that much is actually needed.  If you were wanting to work with hundreds of tracks, and/or using MIDI to do major symphonic mock-ups with high-end sample libraries (VSL, EWQL Hollywood Strings, etc.), then something more would be needed.  In general, high end sample libraries require more RAM, while programs/plugins need processing power.

For example, VSL's MIR is a somewhat specialized program used for hosting high-end, typically orchestral, sample libraries when doing orchestral MIDI mock-ups.  The program is used in conjunction with Sonar/Cubase/Logic, etc.  One would not really try to run that program on anything less than a mid-level i7 9xx quad processor, and 24 gigs of RAM (MIR SE is not quite as demanding, but still really requires an i7 9xx quad core, and at least 12 gigs of ram, though 24 gigs is still better).

> What are the big things a quad core does effectively that a dual core cant?

The simplest and most accurate answer to that is "nothing".

In a more detailed sense some things will be a bit faster on the quad, and some types of apps will work more efficiently on the quad.

Sonar is good at using multiple cores and threads, fortunately, so there is value to be had. It basically translates into things like: bouncing, freezing, and exporting will probably be quicker. Potential number of plug ins will be higher. Stuff like that. Also, depending on this or that factor, you may be able to get lower latency, though this is not a cut-and-dried thing.

But there's nothing you can do with a quad that you can't do with a dual core.

I've got a quad core processor sat on my shelf here, which I'm going to install when the current batch of work is out of the way. So maybe in December or something like that. I'm quite looking forward to the boost I'll get from it, but I'm realistic about it. Some things will be somewhat quicker and more efficient. It's certainly not double the power or anything.