How much user-available memory do you need? How much user-available memory is needed in an android phone that runs Gingerbread and is likely to be upgraded to Ice Cream Sandwich in order to be able to install and work with a reasonable number of apps. 

Of course, the answer might differ depending on the usage profile of the specific user so 2-4 number ranges would be useful (e.g., light, typical, or heavy user).
How much more /data space does ICS need in comparison to GB?

Easy answer: The same.  

3rd party installed apps need as much storage on both GB and ICS and maybe ICS' internal apps need a little more storage, but that's negligible. Most often it's just the user apps that grow bigger and bigger by time (with bigger devices. It's the same with bloated websites as connection speeds grow).

I have installed ICS on all of my current devices and can say that it's not a big difference. I typically need 400-800MB with some games (MaxPayne, GTA3, etc.) and consider myself a medium-to-power user (with number of apps installed).

Breakdown for my Nexus S: 650MB used  
Internal stuff, summed up: around 100MB (after some usage time)  
User installed apps+data: 550MB  
Dalvik-Cache: 85MB (15MB for internal apps, 70MB for user apps)  
App sizes: 350MB (15MB for internal updated apps, 335MB for user apps)  
App data: 190MB (40MB for internal apps, 150MB for user apps)  
Remaining: 25MB for the FS journal and misc random stuff  


So I guess, you need around 80-100MB for the minimal stuff (maybe less) and on top of this all that you want to install.

I think the comfort zone starts at 250MB if you move apps to the sdcard and 500-750MB if you won't.