I wouldn't go overboard with a video card on a 6400. You will be limited by how much information the 603 can send across the bus and something like a Radeon 9200 will only get bottlenecked waiting on the CPU. Remember that all of the G3 machines and some G4's still sold with variants of the ATi Rage chipset and they had much faster buses so they should be sufficient for a 6400 for most purposes. If a high end Rage card still isn't enough, try a Radeon 7000, but I doubt you'd see a big speed boost from anything more powerful.