I built a reverse sleeper PC for Windows 98 games, but with excellent backwards compatiblity for DOS games.

The Geforce FX series has no issues with DOS games, is the last generation to still feature Win98 Legacy 3D features Table Fog and 8bit paletted Textures, but at the same time the first generation to support DX9, so the absolute sweet spot for Windows 98 and earlier. Leadtek's A350 series is of outstanding quality and this entry-level XT can easily be overclocked to 5950 Ultra speeds but it doesn't make sense because...

The RAM is a major bottleneck. SDR-133 is about half or even a third the speed the CPU and GPU would be used with normally.

But the system isn't meant to set speed records. I've got a 2003 system for example if I actually need more speed. The CPU was chosen specifically because it can be slowed down in software. I use SETMUL to throttle it to 400 MHz if need be, and the L1 cache can also be disabled.

The sound card is my absolute favorite for DOS, it is bug free, has very little noise, excellent General MIDI with a patch set straight stolen from the Roland SC-55, but additional Yamaha and Hoontech sets can be loaded instead. The Crystal CS4236 is not my favorite FM synthesis implementation (not even my second favorite, I prefer Yamaha OPL3 and ESS ESFM), but it's actually close. Most people complaining about Crystal FM are talking about the outright broken CX/CS 4235 and 4237, and on this card it can be enhanced with effects like chorus and reverb.
It is a real hassle to configure, but once set up properly, it sounds absolutely amazing. I do have heard that it would not quite sound like a real SC-55, but I own one, and the description of the people claiming that sounds very much like they forgot to play around a bit with the equalizer and add reverb (the reverb on the SC-55 is 100% part of its character).

While building, I played around with giant copper coolers mainly for aesthetic reasons, but also in order to cool the whole thing semi-passively with only the case fans.

If I had another, slower Gefore FX card, ideally a 5600 or 5700 non-Ultra (because they are still relatively fast but need no additional molex power), I'd probably use that, especially if I had a big copper cooler for it. But the reality is, I only have a bunch of FX 5200 and FX 5500 cards, so I went with the 5900 XT anyway. It doesn't hurt to waste the extra performance after all. In a system without legacy feature support, I'd prefer an ATI Radeon 9000 series card anyway.

Specs

Motherboard: Epox 8KTA3+ w/ aftermarket dual heatpipe copper heatsink for the northbridge
CPU: AMD Athlon XP-M Thoroughbred-B @1667 MHz (Geode NX 1750, binned to run at 1.25V / 25W max TDP)
CPU cooler: Thermaltake The Tower 112 (passive)
RAM: 2x512 MB SDR-133 w/ EKL Alpenföhn Ram(m)bock dual heatpipe coolers
GPU: Leadtek Winfast A350XT TDH (Geforce FX 5900 XT 128 MB)
GPU cooler: Zalman VF-1000 LED
Storage: IDE2SATA, Gigabyte 120 GB SATA SSD
Sound: Terratec Promedia Audiosystem EWS64 XL w/ front panel
PSU: Thermaltake "Berlin" 530W passively cooled
Case: Fractal Design Define R4 Black Pearl w/ tempered glass side panel
Case fans: 5x Noiseblocker NB eLoopX "Bionic" 120x25 mm w/ ARGB
OS: Windows 98 SE w/ patches for RAM etc.

Issues remaining:

21st Century Pinball games (Pinball Dreams / Dreams 2 / Fantasies / Illusions / Mania): Black Screen immediately after start. No music playing, no reaction. If run from the Windows command prompt, I can still ALT+ESC out of the prompt and kill it. In MS-DOS mode, only a hard reset or power off works.

Impulse Tracker 2.14: The software works but somehow the screen is streched partly outside of my physical monitor. I think roughly the lower fourth of the screen is outside of it. Other than that it works fine, and this might be an issue with the monitor? But on two other retro systems, this does not occur with the same monitor, so it might also be a driver issue.