As always, the issue would be whether or not you could find drivers for it.
Not being familiar with IBM's PC server lines, if the system came with some version of Windows on it, then your odds are very good. If it was one of IBM's AIX servers, probably less so. The "best" OS would be whatever shipped with the system, but that doesn't mean you couldn't manage to get something else to run on it.
What you may want to do first thing, is go to the IBM website for this particular system, and see if they offer any driver downloads for Windows. If not, then you can try to inventory the hardware in the thing and see if you can find the necessary drivers, but you probably won't have much luck. Worst case, there's likely a pretty good chance you could run Linux on that thing.
I have acquired an IBM x336 eServer-883711U Server. Understanding the unit does not have a sound card, or a standard PCI-e slot, 2 Xeon single core CPU's, would it be difficult to install Windows 7 on this unit and use it as a PC? If not Windows 7, what would be the best opeating system to use with this server?