OpenGL is a terrible APi unless you have ALOT of experience in it... and the added annoyance that you have to really program each card type individually.
DirectX is the best for overall support, not to mention it isn't just a graphics APi but an entire system APi - for controling every peice of hardware within a Windows based computer.
Anyway, anyone who is serious about playing games would've upgraded from Windows Network 4 long ago ... as it is seriously bugged and really not for home use, and people at work shouldn't be playing games whilst they work. Your there to do a job not play about.
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DirectX 8.1 support Windows 98, Windows 98SE, Windows Millennium Edition, Windows 2000 Professional, Windows Server 2000, Windows XP Home, Windows XP Professional, Windows Server 2003, Windows "Longhorn" .Net
DirectX 9.0 supports all of the above, however has ALOT of known problems under Windows9x systems ... Windows98 & Millennium Edition have known stability problems oftenly preventing devices to be created or even detecting that your using version 9.0
which can be frustrating at times.
Windows 98SE seems to be unaffected by this bug for some reason though.