You should test older drivers before GPU swap, but in general - there is a reason why some simply don't use WinXP's last supported generation of hardware for retro PCs.
Some games simply need fixes to work properly on modern hardware, example of those is "Pandora Tomorrow" : https://github.com/Joshhhuaaa/EnhancedSCPTVersus

Right, but are there examples of other games or features that don't work on the newer cards?

What I'm planning to do is basically just have two installations of XP: one that uses the 780ti, and then an older one for the 7900, using the appropriate drivers for that card. The 7900 would likely be used for anything from ~2004 and before.

The only XP-era games I've personally run into with any issues so far, is Republic Commando and broken bumpmaps, but that thankfully can be fixed with a community patch. I run a Titan X on my rig. The impression I get is that most issues with newer GPU's stem from the changeover to unified shader architecture, that started with GeForce 8.

Sims 2 gets a dishonorable mention, because it's not actually broken, but is programmed with a list of explicitly supported GPU's. Meaning one has to dig around in the config files or use the graphics rules maker tool to get it working properly on later GPU's.

If you are willing to put in a little work you can get very nearly any game to run on very nearly any modern hardware.

Two approaches - play your games on the machine that seems to work best for you. Build the computers you want to play with just because it's fun 😀

Scrapland is a game that has a "best" driver and GPU AFAIK. But I believe what worked "best" was ATI x1000 or 2000 series.

For best compatibility, start with a HD 7000 series Radeon with 1 GB of VRAM and the iCafe driver. Write down all games that don't work and list them here. Also, you can make functionally the same build with an i3 and 4 GB of RAM. Also, the XP era support covers DX1-9 and Windows 98 was still being supported in 2004 gaming, so there's no clear line for an XP era.