I just logged into my Plesk admin panel and I see in the statistics that 500MB of the memory is under "Cached" and under "Used" there's around 900MB, only 70MB is under "Free". What's up with that? I have 1gb memory on the server.
No need to worry with cached RAM...it's simply used to store frequently accessed files in memory for better performance. When the server runs low on ram, its freed and used up.