Update:
I have decided to have 2x Retro-PC permanently and several others temporary (discover how it feels then sell it).
1. Asus CUSL2-C, P3 1Ghz, 512MB, FX5600 AGP, SB Audigy 1, Win98SE, WinXP
2. QDI BrillianX S1 440BX, P3 450, 768MB, V3-2k AGP, AWE64, PicoGUS, Win98SE, WinXP => could add an Yamaha 724 PCI as the QDI has SB-link (PC/PCI)
The experimental rigs I am currently planning only for experience reasons and maybe later sell it again:
3. QDI P5I430TX Titan IB+, P 100 Mhz, 32MB, Elsa S3 2MB PCI, FreeDOS => PicoGUS from (2) could be used here
4. Asus P5A rev 1.04, K6-3+/400, 64MB, V3-2k AGP (bought cheap a 2nd one), FreeDOS, Win98SE => AWE64 from (2) could be used here
5. Asus CUV4X-D incl. 2 x P3 1Ghz (got it for 110 € with CPUs) => the seller offers me to buy his remaining 4x512MB SDRAM, but I am hesitating. Even with Win2k, Win2k3, Vista32, Win7x32 I have only a test laboratory. In 2000 there were no games supporting 2x CPUs?
I will surely remain with number 1&2 and will decide what to do with rig 3, 4, 5 in the distant future.
What is astonishing:
currently a lot of people are selling on ebay.de some Voodoo 3 PCI and Voodoo 4 AGP graphics.
The prices for V3 PCI are much higher than V3 AGP, but AFAIK plattforms based upon Intel TX are too slow for a V3 PCI => do you know an use case where a V3 PCI is suitable?
I have also noticed the exagerated prices for alternative 3D graphics Permedia2, PowerVR(Kyro2), Rendition Verite 2200, S3 Savage4 Pro.
# w98SE . P3 450 . 768MB . QDI440BX . V3 AGP . 3dfx 6MB . AWE64 . GUS Pico 2.0 . 80GB HDD
# wXP . P3 1G . 512MB . ASUSCUSL2-C . MSI FX 5600 . 3dfx 6MB . Audigy 1 . 80GB HDD plan
# w8.1 . i7 4790K . 32GB . ASUSH97M-Plus . GTX 970 . AC97 . 1TB SSD . 6TB HDD