Simple: Keep an eye on your memory usage, and pay attention to peak values. Once you know that peak value, then you know whether you have enough or not.
i.e. As long as your peak values are 2GB or less, then your intended configuration is adequate.
If you currently use more than 2GB, then what will happen is your system will need to page data to/from the hard drive, in order to run everything. This is several magnitudes slower than if you had enough RAM. And therefore there will be a performance impact.