My 9800 Pro is the loudest thing in that case, so it and the Athlon XP deserve each other. That said, mine is paired with a Northwood 2.0, and if I had to pick one, I would pick the one I picked, because I'm picky and don't really care for AMD during the K6 / Athlon years. To me, the P4 era is quintessential XP. 98 goes on the Pentium II or even III.

Next question. 😁

Back in the day I had an AXP1800+ paired with 4200Ti, so that sounds sort of natural to me.

Not sure why you would choose two retro systems so close in performance (the P4 will beat the AXP in most benches, but not by a huge margin) though. there's probably nothing that will run great on the one and not on the other...

Just what I happened to have, both gifted by friends that upgraded - and one more, my old Athlon Tbird on an Irongate chipset - my original 98 system, so the 2 best machines that can take the cards... maybe I should test them and sell one of the cards on - as you say, not much blue water between them - Kind of working backwards through my relics, as they've been sitting unloved with Geforce 2MX cards that they had to start with.
Quite appeals to create the setup I would have loved back when I had the 2MX in mine.

Also got a P4 640 that had (originally) an X740XL, but got it running Win10 with a better card, that would pack the most XP punch, but it actually runs Win10 with amazing adequacy (2GB RAM).

My other relics include a Pentium MMX (my Win3), a Pentium, and finally a 486 VLB (yes, I must do the battery on that!).

4 other system are my "modern" kit, though I do count a Core 2 among them.

Leaning towards making the Athlon a 98/DOS with the Ti4200, and either the VIA sound with the legacy compatibility enabled, or maybe a SB Live.

The Dimension, with the CPU upgrade and the 9800 Pro, maybe an XP system, though 768MB RAM will truly give it that legacy XP experience - though it was originally kitted with just 256 (painful!). 768 being a pair of 256 added to the original pair of 128, and not being worth buying another pair of 256

Yeah, that ought to be good. I ran XP on a P4 for a few years with 512MB. It was enough for most of the games I played, though it hit the swap file when exiting back to the desktop. I ended up doubling the RAM when I upgraded to a Core 2 Duo because I had a smartphone and a camera that both shot video, and wanted to do a little editing.

Some day I will look into retrofit heatsinks for the 9800 Pro. That sucker is just too loud for my taste and I know we can do better.