+1 with Skyscraper if they're all in good shape. One advantage to the Kx333 (and older) is they support universal AGP, so you can go with the Voodoo5 + AthlonXP. IIRC the KT400 is where that ended, and they went to 1.5V keying like the nForce and most Intel P4 boards. I think there's a few members around here with KT333 boards and 3dfx cards - might be worth skimming the retro PC thread or something for ideas. I have a KM266 board; it's basic, stable, and provides universal AGP. Performance in that board with an XP 2000+ is better than my 2GHz P4, which would be more than good enough for ~2001 (my P4 system was brand-new in late 2001, so it truly is "era accurate"). AthlonXP is more of a 2002+ thing, but honestly I'd cheat and go for the T-bred 2200 over the T-bird. I say this because you get SSE and other internal improvements (it has higher clock-for-clock performance), and the slower XPs tend to use less power/run cooler than the high-end T-birds. The Palomino may be a good compromise if you want something that was released in '01, but still has SSE and other improvements.
Graphics-wise, the GeForce 2, 3 or Voodoo5 would be a fine choice any which way. The Voodoo5 will be likely much more expensive than GeForce, so unless you either have personal motives for owning 3dfx hardware (e.g. nostalgia), or you need Glide support, I'd probably go GeForce. Other advantages for GeForce include the potential for DVI support (not all GF2/3 cards have DVI, but finding one with DVI isn't as hard/expensive as finding a Voodoo5 with DVI), TnL support (on GF2), and DX8 support (on GF3). Depending on what games you're looking to run, and what monitor you need to plug into, these considerations may or may not be a factor.
On the RAM, I'd just max it out to 512MB. It won't hurt anything, even if a lot of games don't really need it. It wasn't inaccurate in '01 either - just expensive. RAM is cheap today though, so why not? It can also be a benefit if you ever want to run 2k or XP on the box (with 512MB of RAM and the other specs you've mentioned, this thing would probably be setup pretty well for dual-booting 98SE and XP assuming you have enough disk space for both).
Sound-wise, either SB Live!, SB Audigy, or a non-Creative card (e.g. Envy24 based). I'd say go with whatever supports the games (e.g. do you want/need EAX, A3d, etc) or connectivity you need (for example my '03 machine "cheats" there and has a somewhat newer Razer soundcard because I needed optical digital out, and that's what I had already).
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