a fine system, that xp3000 should be a good match alongside gf 6600 for games of year 1998-2004

there are workarounds for win98 large ram, to be honest i doubt anything you run will need more than 1gb at most even on xp. I'd leave it at 1gb for any 32 bit windows XP system anyway, a 32 bit application can't address more than 2gb of ram anyway and i don't think any game from back then had requirements for more than 512mb (iirc)

there are, i believe, a few win9x games that wont run well on XP even with compatibility mode - a search will reveal a list, and most of these have patches and workarounds so unless a favoured game was on the troublesome list then I wouldn't even dual boot in this case

oh, and system pics when done are always welcomed 😀

Some observations from my similar era PC.
P4P800 End of Win98 Support Build

Optical drive.
I still prefer installing XP with a real CD, once the OS is up I use virtual drive but keeping the physical drive doesn't hurt.

HDD.
2TB is huge for XP let alone 98, You'll need to partition it at the very least but may still run into problems with LBA48
RLow had a patch for drives larger then 137GB
http://lonecrusader.x10host.com/rloew/patchata.html

but if it was me and your creating partitions anyway I'd just use 2 drives, a smaller one that doesn't need work arounds for the 9x and XP install and a larger one for XP's "data" where the limitations wont matter.

OS
Even though I built it as a stupid fast 98 PC I mostly used XP.
XP is more stable so would only drop back to 98 if needed and any game that needed 98 didn't benefit from the extra speed over my P3 anyway.
Not to mention the GF6 isn't the most compatible card for 9x anyway.

Ditching the X1600 Pro for the HD2400 Pro wasn't the best choice. The X1600 is the faster graphics card by around 50-100% depending on the game and the settings.
It'll really struggle in the later part of your targeted era (2003-2004 games). I'd recommend keeping your eye open for a 7600GS at least, but even that will struggle to maintain 60fps in Far Cry on very high.

I still have my VisionTek AGP HD2400 Pro 256MB that I bought open-box in like 2007-2008. I ran it with a 1.8GHz P4 and 1 GB of RAM at the time. I actually ran Windows 7 RTM on that card, although it required a hotfix driver to work. I will say it was significantly better than the PCI FX5500 it replaced, but that's not saying much.

Watch out for the 5V rail on your power supply. I expect your choice of CPU could be a problem: the Athlon XP (and the vast majority of Socket A motherboards) usually uses 5V to power the CPU. Modern computers primarily use the 12V rail, and thus modern power supplies deliver most of the power through the 12V rail. Many modern power supplies will not boot a socket A system. The Athlon XP in particular are some of the fastest and most power hungry CPUs prior to when the industry switched to 12V (Pentium 4, Athlon 64). You can read about this at this good website and this thread.

In regards to SSDs, both Windows XP and Windows 98 lack support for TRIM. For older SSDs that was a problem, you'd get performance degradation over time and eventually decreased longevity of the SSD. There are a few workarounds:

• Some specific early SSDs (Intel) come with a manufacturer's specific software which works in XP. Their specific software detects the compatible SSD and allows you to manually TRIM.
• Newer SSDs (~4 yrs or newer) have advanced garbage collection built into the firmware so you shouldn't need to worry about the OS not performing TRIM
• You could ignore the lack of TRIM under the assumption that a retro PC gets relatively little use so the inevitable problems wouldn't occur for many many years
• You could use a traditional hard drive, or a hybrid drive (eg Seagate Momentus XT).

I see that you've now decided to skip Windows 98. I agree that dual booting 98 with XP is kind of a hassle, especially since XP performs better with more than 512MB RAM. In my experience the selection of Win98 games that don't run on XP are somewhat limited (some definitely exist), especially if you are willing to find/use patches that allow some of those games to run in XP. Windows 98 is handy for DOS games, which is not what you're going for with this build.