Neither. spend the $400 to go towards a new motherboard, new RAM, and a new CPU. if your still usign AGP 4x, than that means your using old parts, probably later P3's and earlier Athlons. i'm guessing in the 800MHz to 1.4GHz range. An X800 or a 6800 will do little good because the rest of your system won't be able to keep up.
If you use that $400 to go towards an entry level PCI-E system with say, 1GB of RAM, a sempron or an Athlon 64 3000, a X800GT, 6800 vanilla, 6800GS, or a 6600GT, it will perform faster than your current system with an X800 or a 6800, garunteed.