Like I said, it depends on what you want to play. If all you want to play is 2D games or software rendered 3D games or even early D3D games at 640x480, then the onboard IGP is probably all you'll ever need.
That Pentium 3 is pretty fast, but PCI cards are rarer and more expensive than AGP counterparts. Your best bet is one of the following:
Voodoo3 PCI
Riva TNT2 PCI (not M64, these are much slower), I think they made a PCI version of that
GeForce2 MX PCI (MX 200 was quite popular)
GeForce 4 MX PCI
GeForce 5200 PCI
I think the GeForce MX cards are the most common and they will allow your system to really flex its muscles. I'm not 100% sure that the TNT2 ever saw a PCI release, avoid the TNT2 M64 at all costs, as it is essentially as fast as a Riva TNT (not much faster than your current card). The Voodoo3 is expensive, expect to pay 60-70E for one, but I thought I should mention it in case you wanted a 3Dfx card. There's also the Voodoo5 5500 PCI, but that's on a completely different price level 🤣 .
I don't know if ATi also had PCI versions of their cards at that time, I guess some of the Radeon cards could be nice additions as well.