Can I do this on an iPhone

Am considering getting an iPhone/iTouch. I have heard that it can be used to control iTunes. I have never seen or tried this, so I have a question.

I have 100 GB music stored on an external HDD. I connect wirelessly to the eHDD via my MacbookPro and stream to my stereo via Airport Express.

So, does iPhone basically have a copy of my iTunes library and I control my MacbookPro via iTunes? This would require the computer to be on.

Or, does iPhone communicate directly with the eHDD and stream to the AE? This would mean I can leave the computer turned off.

How about having a few songs on the iPhone? Is it a separate library?

Basically, it would be nice to be able to control all my music at home via an iPhone (preferably without the computer turned on), and to have a few songs on the go too.

The iPhone controls your iTunes library via iTunes on your Mac so your Mac has to be on. It's effectively a wi-fi connected remote control.

There's no processing 'intelligence' on your external drive for your iPhone to communicate with so you can't do this with your Mac being powered up.

There are solutions to send music from your iPhone to wireless speakers. Over bluetooth/wifi but all the one I can think off rely on sending music that's present on your phone not on another drive.

Thanks. Now I understand the gist of the application.