memory footprint and resource planning

I have an inquiry about possible setup scenarios for different use cases. I started having a single image installation at first. Then I collected some pier instances. After that I wanted to separate installations for setup and security reasons. I didn't know about multiple stone setup that time.
The most feasible way seemed to have multiple users in one image that fully bootstrap the complete system. The advantage is having only one set of the central processes of a gemstone installation that can share the shared page cache. The drawback is that for small installations the bootstrapped system classes outweigh the data that is being collected. So the effective sharing benefit of the cache is doubtful.

Talking to Andreas Brodbeck I learned about multiple stone setups. Thinking about it for a while it appeared to be better for several reasons. The installations are really separated, can be constructed/bootstrapped in there own workflow without affecting the other installations. But now I have some concerns about the memory usage I'll face having quite some installations. I read the slicehost article from James where he installs a gemstone in a 256MB slicehost instance. In my case however a gemstone installation with a spc of 200MB eats easily 500+ MBs after boot up. So how does this fit with the slicehost installation? Is the gemstone grapping more memory as long as it is available? Or are there some parameters that can tune the consumption of gemstone? I think slicehost is comparable to the setup I have using openvz which does not allow memory overcommiting.

Can you give some numbers about the normal memory consumption to expect from a single installation? Would it be safe to set some limits if memory growth is dynamic? What is a feasible size of a temporary object memory for a gem? At the moment I need to calculate for two stones

- 2 * 600MB stone + spc + support gems
- 10 * 70MB for webserver gems
- 250 MB for mfc gem
- 70 MB for a seaside session cleaner

that gives a total of 2220 MB for this setup.

thanks in advance,

Norbert