Hi Bazza,
WordPress Low Memory Limit
First of all, I don’t know if you can put 1G. Probably you need put, 512M, for example:
define(‘WP_MEMORY_LIMIT’, ‘512M’);
I have some big sites and I never put more than 512M. Try to begin with 512M.
Here is the link with WordPress official info:
PHP memory limit
Second thing, The PHP memory limit needs be bigger than WordPress Memory Limit and not bigger than your Hardware Memory.
You can find details here:
http://wpmemory.com/php-memory-limit/
We have this plugin to help you to manage the memory.
https://wordpress.org/plugins/wp-memory/
If you have a small site, with a simple theme, our plugin Stop Bad Bots can works fine also with 40MB of WordPress memory Limit.
The total memory necessary depends of your theme, how many plugins, page content (images and so on….). Then, if you install more and more plugins, change to a heavy theme, use big images, slides, you need more memory…
We explain How memory Your Site Needs in this page:
http://wpmemory.com/how-much-memory-enough/
Please, let me know if you have questions.
Cheers,
Bill