I have a test VPS with Plesk installed and most of the time its using below 150mb. But I tweaked a lot of things though to reduce the memory usage. Maybe you should try disabling the more memory-hogging features.
Parallels offers a minimum requirement of a 266MHz Processor, 512 MB of RAM (768MB minimum when using the site builder, 1G of ram when hosting "many" sites), and 10G of hard drive space for a Linux server. I can say from experience that a Debian 5.0 Linux server (albeit 8x cores strong), with 140+ clients on it, is using about 1.5G of RAM.