It's a difficult bridge to cross. WinXP means dx9, and cards with good dx9 support often lose compatibility with win98. I would say a geforce FX 5500 (fairly cheap), but it is not a strong performer. FX 5900 Ultra is better but expensive. You are better of with 2 PCs, one for 98 and one for XP.

It really depends on how smooth you want your WinXP gaming experience to be.

If you're aiming for a locked 60 FPS at 1280x1024 then I'd say go for a separate XP system with a more powerful GPU and a faster CPU. But if you just want those early games to be playable at around 30 FPS and/or don't mind using lower resolutions like 800x600, then that Pentium4 and a GeForce4 Ti 4600 might be enough.

GeForce 5900XT. Cheap, fast, available.

What kind of sound card are you using? I recently was reminded of the issues with running Aureal Vortex cards with anything greater than DirectX 7.0a. Also had trouble exiting DOS boxes with an FX5500 in 98se with newer drivers and DX9. 0c. For a less problematic experience I definitely recommend using older stuff for Windows 9x.

GeForce 4 Ti4600 should do the job unless you need DX9 but that is really not early Windows XP stuff.