It's worth noting that "Vista's RAM eating capabilities" are overrated. Vista does something very sensible: It uses the RAM if it's available. If the RAM is otherwise unused, why shouldn't Vista use it to cache recently used data? You don't gain anything from having large amounts of unused RAM. And of course, if your applications start needing more RAM, Vista will start releasing some of what it's using. So Vista isn't that bad.
Apart from that, it depends on your usage. How much is the harddrive used? How often are applications launched, how often do they need to access a lot of files?
For a lot of people, the usage pattern is something like this:
- Boot: lots of harddrive access
- Launch my applications: Lots of harddrive access
- Work: Almost exclusively in RAM
And usually, almost all the time is being spent on #3.
Occasionally, the antivirus scanner may choose to poke at a few files, or Vista will start indexing the harddrive, but in general, once the system is up and running, RAM becomes the most important aspect. But of course there are areas of work where this isn't true, so it depends.
On the other hand, how much RAM do your users need? Most applications run fine with 2GB RAM. Even on Vista, and even with a handful of background processes. If the system will never need more than 2GB, plugging in 4GB is rather pointless.
Ultimately, it depends on the software you're putting on the system, and what the users are going to do with it. We can't answer that one for you. :)