If you want to play in 3D games, I would recommend a voodoo. We've been waiting for a radeon 3D driver for a long time and it doesn't seem as if we're ever going to get one. Needless to say, running out and getting a GeForce is useless, as there will never be drivers for any platform except Windows/x86.
Edit: DVI is supported, at least on radeon. I was just told by several users, as well as JoannaK.
You don't need a sound card or a network card, so leave those out. Those are integral to the Pegasos. Same goes for USB and firewire.
If you're interesting in playing old Amiga games, a Catweasel 3 PCI is useful for reading old Amiga disks, and you can also use Amiga keyboards and joysticks, and even play C64 music with a real SID chip (no driver for the SID and keyboard yet, though :-()
A PCTV card is nice, but hardly necessary. You'll have to pick it carefully though for support. Same goes for SCSI.
ATA100/133 hard drive with correct ATA100/133 cables is essential, as is a modern CD-ROM. The older cables might not work.
And most of all...wait until you have your board first before buying any of stuff. For now, just research it.
