3dfx was amazing at the time, but not so much now. The list of DOS (possibly a *very* small number of Windows?) games that work only with Glide is remarkably small.
Modern hardware - struggling to think of much, really. The VR helmets coming out should be significant, as possibly may be other stereoscopic 3D technologies - 3D vision, and passive stereoscopic monitors - I love my Zalman. It may also be worth having a monitor which is not HDCP enabled.
Possibly a fast Core 2 system, before Nehalem hit and changed the architecture to add an integrated memory controller (ok, AMD got there first) and SLAT (EPT/RVI). Core 2 and certain chipsets have the now rare ability to do VT-d without SLAT, and independent of CPU capabilities.
Newer Core 2 is also the point at which PATA disappeared, although who cares in these days of SATA to IDE converters?
The parallel port, and often the PS/2 ports, are usually MIA in new motherboards. Oh yes, floppy controller, too.
New systems are mostly UEFI based, for those who care. I can't imagine many people will want to bother with the early EFI implementations - they're useless for almost anything other than updating the BIOS and some very basic testing, but you never know.
Many modern systems are now entirely PCI-e based. There are older systems supporting PCI, and PCI-X.
Specifically it's probably worth keeping hold of 975X chipsets (fast, 8GB memory, supports everything from old pentium D to quite new Core 2), and some of the Q series chipsets (for VT-d). The Intel DX38BT and DX48BT are notable for supporting Core 2, PCI-e 2.0, DDR3 and VT-d and it *actually works* (supposedly). The S3200/3210 server chipsets, which I have a lot of experience of, also work properly but have so many caveats it's not funny.
I can recommend exercising caution with the HP xw4600 motherboard. On one level it's lovely, with DDR2 support, lots of Core 2, all the ports and PCI-e 2.0. On the other hand, it will not work with most power supplies (timing issue), and the VT-d is broken. Also, it's X38 based, so you can't do an LGA771 to 775 hack on it.
I also agree the SR2 is rather special, being the only(?) dual processor Xeon motherboard supporting overclocking. It's so specialist it again has a fair few limitations, and issues, though.