A SCSI card could be useful for using older peripherals. Like I mentioned in another post I paid extra for one in my G4/350 when I bought it new...but never actually needed to plug any of my old stuff into it. For all I know it came DOA 13 years ago and I still haven't noticed!

TV tuner is a cool idea. I wouldn't worry about a card for newer drives until you're ready to try installing a newer and faster drive. I'm not sure about OS 9 compatibility, so I'm interested to hear from someone with experience there.

Maybe a card for additional (or faster) USB or FireWire, if needed.

I think the most useful thing for communicating with older Macs may be a ZIP drive, be it internal or external. (Did they still offer an internal ZIP then?) Combined with an external SCSI ZIP drive they can make transferring files to and from old Macs much easier than jumping through networking hoops. Again, only if you need to do this. I find joy in networking computers, especially when there's a challenge getting them to talk and a little creativity is needed. If you just want it to work, ZIP will bring you from new all the way back to a Mac Plus.

How about another video card for dual monitors? (Although, why stop at only two... } )