PHP memory for WPMU

I'm doing a research about which company I should choose for hosting, and in future moving to virtual personal servers and then maby dedicated if necesairy...
I'm gonna have a network of sites along with Buddypress.

I checked different hosting companies and I found out that I don't have to be botherd about bandwidth and storage cause most of them they give ulimited amount. The thing i need to be aware of is how much php memory I can have an acces to, in order to run my servis.

I came up with my 3 main choices:
1. HostGator up to 256mb php memory
2. BlueHost up to 128mb php memory
3. DreamHost (don't know yet)
This is all for shared hosting (good for the beginning)

I'm trying to think ahead as much as I possible, and theoreticly if I would like to moove to personal hosting I found the best option with DreamHost
cause you choose your plan based on php memory. This is the only one were you still have unlimited space and bandwidth.
Has anybody have/had any experience with DreamHost? Do you think it might be a good choice (thinking long term)?
Does anybody have an experience how much bandwidth and especially php memory can be consumed when having (lets exaggerate) around 40 plugins and lets say 100 sites within a network.
Any advice articles or link will be helpfull. I'm trying to visualize the bussiness in order to plan some moves.