Welcome to Android Central! 300 MB is very low. The system has to keep a certain amount of free space available to account for temporary storage needs, so just because you have 300 MB available doesn't mean you can install anything. An app's filesize listed in Google Play is only the download filesize--when it's fully installed, it's usually bigger (just like in Windows), and that might exceed the threshold that the system has set for low memory.
See this guide for tips:
http://forums.androidcentral.com/amb...-warnings.html