Newb looking for suggestions

Hello,
I recently had on old Dell PowerVault 745N fall into my lap. It runs remarkably well, and while it's definitely on the low end as far as power goes (1 GB RAM, 4 250 GB drives, Pentium 4 @ 3 GHz, Windows Server 2003 Appliance Edition), I've only had to replace a CMOS battery to get it ruing smoothly. I thought it'd be fun to try messing around with it, and maybe use it as a multimedia server. I've done a little bit of research and, though I've never used Linux before, thought now might be a good time to try it out. I've ordered some additional RAM and eventually I'll be upgrading the drives as well.

To start though, I wanted to see what suggestions you folks might have as for the OS. It's a 32 bit system, so I'm a little bit limited, but I imagine the server version of Ubuntu is going to be the best fit? Will that work with PLEX? Also, I'd like to try ruing a ZFS system. Is that also going to work with the server version, or do I need to use the desktop version?

Finally, I understand the server version can be run without a GUI, which would improve performance. I'd like to squeeze out every bit of horsepower I can from this thing, but I'm wondering if it's possible to install a GUI as an option. In my head, it's the kind of thing I'd tu on/off with a command line function so it's available if I wanted to manage the server directly, but doesn't run otherwise. Is that a possibility or am I imagining things?

Thanks in advance.