I love Windows Server 2003 and it will do all the things you want with exception to the gaming which it really isn't intended for but can be hacked to work.
I'm actually running on it on the brand new machine now. I was fiddling with the domain stuff, but honestly it seems like a lot of hassle for managing one user (me). I'm not sure it provides any notable benefits over sticking with a peer-2-peer setup on a 3-machine network, so unless I see something that talks me into sticking with a domain, I'll probably go back to workgroups.
Functionally, it not's that far different from XP Pro which I run on my personal machine. It seems a bit more geared towards server use, but other than that, it's functionally identical (though looks more like 2k). Getting my 3d card working didn't take any real effort -- I don't think it's that 2003 doesn't support gaming so much as it's disabled by default for obvious reasons. I did have to install the XP drivers for my Nvidia card which seemed to function just fine, and turning the 3d hardware acceleration on seemed to fix the gaming issue, though admittedly I haven't actually tested it out any gaming yet -- haven't really had the time.
I guess I'm mostly just fishing for whether or not it may be better optimized for dual cpu (or in this case dual core) performance over what XP is, or if they're essentially the same in that regard. I was hoping, depending on responses, that a clear-cut choice would show itself so I could install one and be done with it. I guess if I had to choose right now and performance was nearly equal, I'd probably still go with 2003 simply because I'd have some other functionality waiting on there server-wise if I ever needed it, that XP may or may not provide.
For reference, my setup is basically 1 file server (which is what we're talking about in this thread), 1 personal gaming/development machine, and 1 media server. The file server has a raid 5 setup on it, that will mostly be feeding files to the media server so I can keep most of the hot and loud hard drives remote from the HTPC.
I have licensed copies of both 2003 standard and XP Pro here already which I obtained via Microsoft's Partner Program through their Action Pack subscription, so this is purely a question based on which will perform best, rather than a most bang for the buck sort of question.