The min required (as stated in the editor) is 512mb. If it doesn't work, then 90% of all problems are caused by a lack of RAM.
Also, if you have 512mb, but an onboard graphics chip (ie motherboard based), then a portion of your RAM will be reserved for graphics processing (even if it's not needed).
With a 512mb system configured to use 32mb of RAM for graphics, you're looking at only being able to use 480mb.
I think the majority of people would advice you to upgrade your RAM as it's one of the simplest hardware upgrades out there.
For more information on upgrading your RAM, I would go to
http://www.crucial.com. They have an excellent utility that you can download and run. This will then tell you the correct RAM type you should buy for your system (note: not all RAM chips will work for your system) and offer you the chance to buy it.
As a guide, 256mb sticks of RAM are currently (for my desktop) £20-£30. You can pick up different brands of RAM, and I'm led to believe that brand really can affect quality, but since I've done no real research into this, I can't tell you if that's true.
At the end of the day, most modern systems would be quite happy running on 1gb and you should aspire to get that as a minimum as RAM is the most common (and cheap) speed boost.
Hope this helps some people.
Pre-Game Editor 
Linear Mode
