I have a computer with Windows 8 Enterprise (Evaluation) and an Intel Q8300 CPU (which does not support SLAT as far as I know).
I also own a Samsung ATIV S - Windows Phone 8 smartphone.
I would like to develop Windows Phone 8 app(s) but the emulator won't run on my PC. What are my options? Can I plug the phone via the USB cable and debug the app on it? Or this will also require a developer account (which I hear is 100 US dollars per year)?
You have to unlock your phone to debug applications on it, and yes, you need a Windows Phone Developer account to do that. It costs 100$ per year.
MSDN: About deploying and running apps on a Windows Phone device
You have to meet the following prerequisites before you can deploy an app to a Windows Phone device:
You must be a registered developer. For info about registering as a developer, see Registration info.
The phone must be registered. For more info, see How to register your phone for development.
And about Dev Account cost: Register for a Dev Center account
An annual Dev Center subscription is $99 USD plus any applicable tax. For that, you’ll get to submit unlimited paid apps to Windows Phone Store. You can also submit up to 100 free apps.