My case was a tall ATX one (I can find Baby AT cases like that, but they do not look the same) with three 5.25" bays and two 3.5" ones, and the PSU was mounted like in newer ATX cases (on top as an exaust, not above the mmotherboard blowing air into the processor). I'll hunt it down eventually =)
Don't stress too much about Voodoos...This seller put a bunch of reasonably priced stuff up (MercadoLivre.com.br), and boy... his Banshees sold almost instantly. I was able to grab a Voodoo3 3000 and I like it very much. Sharp desktop image and good performance for what I use it for. He's got a Voodoo1 for sale, but at a higher price. That one doesn't look like will get snatched anytime soon (unless I get some extra cash in the near future, that is xD). My Voodoo2, though, was bought from Korea (through ebay) with free shipping, and it was still cheaper than the ones I've seen here in Brazil, but it took a while to arrive. Unless you wanna mess with Glide, though, just snatch a cheap nVidia card (check the voltage regulator if you get an AGP card). MX4000s are cheap, plentiful and will probably offer more OpenGL performance than the K6 can tap into anyway.
Btw, for gaming even a 550MHz K6-3 can't match a Pentium II 450MHz. 3D gaming does use the FPU, where the P2 is much better, and even 3D Now! won't offset this completly (and there are plenty of games that do not support it). Still a fun platform to mess with.