Just a note that for a VPS or shared solution on top of Linux, I would recommend Xen-hosted or Virtuozzo-based solutions.
In Xen, you cannot over-commit RAM, so if you pay for a 256MB VPS, they cannot hide whether or not you actually have 256MB dedicated to you. And 256MB if not 512MB is the bare minimum I would recommend for OpenACS or .LRN if you have any kind of traffic.
In Virtuozzo, the resources are usually shared and over-committed or over-subscribed, however, Virtuozzo has a good reputation amongst hosters, provided that the company running the server knows what they are doing and set limits properly. Also the filesystem cache is not part of your RAM allocation as I understand it, since the filesystem is cached globally by the host kernel.
I have found that Solaris Zones (part of Solaris 10) are superior to Xen in terms of CPU performance and ease of use, and superior to Virtuozzo in terms of quick setup, maintainability and price (especially since Solaris 10 is free). Also Solaris' virtual memory manager appears to perform better than Linux, however, with all the changes in 2.6.x that may no longer be the case. However, aside from Zill.net (my company), virtually no one offers Solaris as a shared server solution.
