I had 3 such machines, two remain (maiboard on my 478 rig died - again).
So first off I have a willamette socket 423 rig - I got this because I'd never seen a 423 P4, or RDRAM before:


It's a Dell GX400. Neat little machine. I replaced the Rage 128 in it with a radeon 8500 and added a yamaha DS-XG - it makes for a great machine for 1997-2001 games. It can run most 1998 and 1999 games at pretty high resolutions. It has 512MB of ram and runs win98se.
Another of my P4 win98 rigs is this monstrosity:


This has an LGA775 3.6GHz pentium 4 660, 1GB of ram and a Abit AS8, a Radeon X800XT and an Audigy 2 ZS. It runs win98se as well. I use this machine when I want to get silly. It runs Black and White @ 1920x1080 with AA and AF, Homeworld and Cataclysm at 1600x1200 with AA and AF, and DK2 @ 1600x1200 with AF. Old games have never looked so good!
The third machine was a 3.2GHz prescott (socket 478) but so far it killed 3 boards - two Asus P4P800-X, and a P4P800 Deluxe. It had a Radeon 9800 PRO gfx card. Needless to say I won't be fixing it anytime soon.
To answer your questions, a P4 or fast Socket A rig could be used to run win9x 3d games like they have never been run before - as much eye candy as the game and video drivers permit.