In the year 2000, an Aureal Vortex 2 SQ2500 was probably the most desirable sound card for gamers. There are a few cards generic cards that use similar Vortex 2 chips though, as well as cards from other brands (like the Turtle Beach Montego II), look them up. Look for a PCI sound card with a Vortex 2 AU8830 chip. AU8820 and AU8810 are the older or somewhat stripped down models. The SQ2500 is blue and square but most other AU8830 cards are small green cards with an odd angled PCB. They stand out pretty well if you're looking at old unmarked boards at thrift stores or on eBay.
You could get a Live! but, meh... they don't really do anything interesting IMO and they are so common that won't feel that special. At least the Vortex 2 will give you A3D 2.0 in a lot of games from that time period, which can't be done with any other cards or via emulation.
For the rest of the specs, I think a Socket A Thunderbird 1.2Ghz or the 1.5Ghz P4 you have are both good options. At the time, enthusiasts would have generally gone with the Athlon since it performed better in most games, but the P4 certainly wouldn't be bad, and Socket 423 is fairly uncommon to find now and would make for a more unique build.
For video, the Geforce 2 Ultra was certainly the king at the time. An original Radeon DDR (no numbers, not a 7200 or 7500) might be interesting too, since they traded blows with the higher end Geforce 2 models but I never had one so I can't comment on how usable the drivers are\were at the time. I remember ATI having a bad reputation back then, but I'm not sure if it was legitimate or just a left over from the Rage days.