We started doing some tests regarding your scenario. We plan to post the results of these tests when we collect enough information.
However, we think that you have to upgrade your hardware in order to support so many web applications (even they are not in Sitefinity). Actually, the recommended RAM for a site is minimum 300 MB.
We just released Sitefinity 4.1, you can try it, because it performs better.
By the way, in Q3 2011 we plan to add multiple site management with a single administration. Maybe it will be the appropriate for you case.
the Telerik team