The Radeon will support Anti aliasing on older games. You can also get G4 zif upgrades, see here.
Be careful though, the G3 300 will have the dreaded Rev 1 motherboard which was notoriously glitchy with IDE harddrives and stories of data corruption and incompatibility abound. Before spending a small fortune on a G4 cpu, it may be better to get a cheap quicksilver G4 which would also give you AGP, a higher RAM ceiling and much overall higher performance.
It's also worth remembering that unless you are using OS X or apps that benefit from Altivec then the G3 is actually a tiny bit faster then the G4.
Be careful though, the G3 300 will have the dreaded Rev 1 motherboard which was notoriously glitchy with IDE harddrives and stories of data corruption and incompatibility abound. Before spending a small fortune on a G4 cpu, it may be better to get a cheap quicksilver G4 which would also give you AGP, a higher RAM ceiling and much overall higher performance.
It's also worth remembering that unless you are using OS X or apps that benefit from Altivec then the G3 is actually a tiny bit faster then the G4.