For all it's worth, here's my 2 cents:
XP covers a huge span of games and systems, from late pentium 3 rigs to the first core i7 - and to make it worse, the performance gap between generations is significant, especially when it comes to graphics cards. Think 8800GTX vs 7900GTX. As such, I found it's best to either stick with higher end hardware from the middle of xp's lifespan or later hardware, from around when windows 7 and windows XP overlapped usage wise. I'm skipping windows vista here because it seems not many people used it and it didn't gain nearly as much adoption as windows XP or windows 7 did.
So my windows XP machines tend to either be high end stuff from 2006-2007 or mid to high end 2010-2012 hardware. These systems have great winXP support and can cover the whole range of games that came out in that time frame and support windows XP / direct X 9 and 10. One of my XP builds is a dedicated XP machine, built around an nforce 780i mainboard. It's running a core 2 quad x6800 cpu with 4gb of ddr2 and a GTX 280. The other dual boots XP and Win7, and is built around an intel X58 LGA 1366 board with an i7 950, 6Gb of ram and (usually) a GTX 580, although right now I have a Radeon HD 7950 3GB in there.
These two systems can run anything from Doom 3 up to Fallout: New Vegas - more precisely anything made between 2001 and 2010. The i7 dual boots Windows 7, so you can extend that to 2015 in that particular machine's case - witch is why I temporarily swapped out the GTX 580 for the 7950.
Sure, you can get away with older or lower end builds, like a basic core 2 duo and a gt 650 or 8800gt, but it depends on how many games you want the XP build to cover and what kind of performance / detail / resolutions you want to game at. I'm saying this because I have friends that enjoy playing GTA 3 on an 1.8GHz pentium 4 with a geforce 4 MX at low resolutions and mediocre framerates, and consider that as part of the experience (weird kind of masochistic nostalgia if you ask me), while I like to max out the resolution / detail levels and have a smooth 60 + fps.
And while you're able to run XP just fine on an athlon XP (I did back in the day, but I dual booted XP and 98), having loads of systems around the house is impractical (I would know), so I went with configurations that can run games in a 9 year time span.
All of the above only takes into account usability. If you enjoy building machines for the sake of it, the PC you found on ebay can be used for XP, although it makes a fore fitting late win98 build in my opinion, as I found XP service pack 3 is a bit on the sluggish side on anything but the latest socket A / 478 hardware, and by latest I mean nforce 2 + Barton core 2500+ to 3200+ or i865 + Prescott 2.8 to 3.2GHz.
tl;dr - XP gaming covers a very big span of games with greatly different requirements. To cover everything, get a later core 2 duo or early i5/i7 PC. If budget is a concern, then a pre-built core i3 540 or 3240 with a GT 650ti or radepm 7770 should cover your needs nicely. If you want to be a bit more period correct, a late core 2 duo/quad e6xxx/e8xxx or athlon II / Phenom II / Phenom X4 with a GTX 260 216c or GTX 280 / Radeon 4870 / 5870 will run any game you want and blow most things out of the water.
Windows XP for Games from 98 to about 03/04. Newest (compatible) hardware is around 2012ish. Directx 9 system.
I don't know about that. Anything that runs on vista will run on XP as well, and it will run better - so I'd push that 04 up to and including windows 7's launch year + 1. Games like fallout new vegas and Warhammer 40k Dawn of War can have issues with modern OS's like win 10 due to the Games for Windows LIVE thing, or various DRMs that are incompatible with win 10 (in some cases win7). Sure, there are patches for them, but I don't like spending hours looking for a patch then testing a game's stability instead of enjoying myself in the little time I have. New Vegas for example is a game released in 2010. That's Win 7 age - yet on my i7 950 rig, it crashes to desktop every 30 minutes in win 7, while being perfectly playable for hours on end in windows XP 32 bit on the same machine.