There is no place where you specify global limit for SSIS processes. You control memory consumption granulary, by controlling specific SSIS tasks memory constraints. A good example is the Lookup transformation where you can limit the amount of memory in use for caching. The SSIS data flow task processes data in chunks, so if you don't use large buffers you will not have problems. I think you have to be more concerned about CPU contention. SSIS can be quite CPU intensive.
I'm wondering why you are not using 64bit OS if you have 8GB of memory?
SSIS Tasks Components Scripts Services | http://www.cozyroc.com/