So I run this server at our company. It has a Core 2 Quad CPU, 8 GB of RAM, as many hard disks as possible etc.
I run Windows Server 2008 on it that runs several virtual machines, one of it is another instance of Windows 2008 that acts as fileserver and domain controller: the domain has about 30 computers and the fileserver is used by about 15 people normally. I gave it 2 GB of RAM but I'm starting to wonder, can it run with just 1.5, or even better, just 1 GB, without a noticable performance drop?
I'd find a much better place for that extra gig of RAM...
I'd say yes, quite possibly yes.
It would depend a bit on the type of file server load, how much it's gaining from caching files especially writes or if that doesn't matter for the usage pattern.
Also, is it 32 or 64bit? The 64bit version will need a bit more memory to run pleasantly. A 32bit Server 2008 or Server 2008 Core could get away as a small-scale domain controller and file server with 512 MB memory, I know mine do...
...it's easy to test though, right? ^^
(I'd personally make two separate 32bit Server 2008 (Core) servers, one as DC and the other as a file server and if you don't already run two DCs - set up a second one. Perhaps the two DCs then could get away with even less memory like 384 MB each in that environment)