top will show you the top active processes use "ps aux" to get a listing of all current processes. You'll also need to watch out for processes that are actually child processes which are sharing most of their code or data segment with their parent processes and also programs which are loading shared libraries which may be... shared with other programs, and thus not fully responsible for the memory used by these libraries.
Image processing software is a known memory hog, though. You should definitely check things out when you're generating thumbnails, uploading new photos, and when you're viewing photos at various sizes. You can run two or three browser windows at the same time and hit control-F5 to refresh without using the cache to maximize the requests to your site.
I've been considering getting a second non-PS plan for my Gallery site as it's one of my personal, non-critical sites.
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