What are my best options for upgrading?

Add as much memory as you can. As you are a music producer and DJ, you need the best hardware and a fluent operating system, so you can erase the disk, add more memory and upgrade to Snow Leopard.

First of all, open  > About this Mac > More Info, and copy Model Identifier here, so we can know how much memory support your Mac and what operatings system supports, but it looks like you can upgrade it to Mountain Lion

You have a Mid 2007 iMac. You can upgrade to OS X Mountain Lion.

First of all, your Mac supports 6 GB of RAM. You can buy memory in OWC. I recommend you to buy 6 GB of memory because you will need it.

Then, I recommend you to upgrade to Snow Leopard, so buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard. Make a clone with Carbon Copy Cloner to back up your data.

To install Snow Leopard, I recommend you to erase the disk if you haven't got enough space on the hard disk. If you have enough space, insert the DVD and upgrade Mac OS X. The disc has got Mac OS X 10.6.3, so open  > Software Update to install the most recent version.

Some Logic versions don't work with Mountain Lion, so I recommend you to keep Snow Leopard. To install the memory > http://support.apple.com/kb/HT1423?viewlocale=en_US&locale=en_US

You are welcome

They are the same on all the countries

You are welcome