Anything IDE will work...
I'm not a hard drive expert, but I'm happy with my Quantum FireBall LM 20GB in my 6400 right now. The only problem with buying an ATA/66 or even ATA/33 drive for a 6400 is that the 6400's max transfer rate is something like 16MB/sec, as compared to ATA/33's 33Mb/sec and ATA/66's 66MB/sec. However, the nice thing is that once you upgrade, you'll have a nice, (relatively) fast drive for your G4 or whatever else is comming down the pipe.
Don't quote me on any of those throughput figures, by the way, as i'm not a hard drive expert, like I said before.
Good luck with your install,