The closest comparison I can offer is A7V266-VM versus A7V333-RAID, both of which I've used in my office system over the last year, with the other parts remaining the same except for the video (onboard for the A7N266-VM, TNT2 Vanta AGP card for the A7V333).
Judging by mine, they're going to perform pretty similarly. If you ran benchmarks of, say, a full virus scan of the hard drive, I'm not sure which would win: A7N266-VM by virtue of great IDE drivers, or KT333 by virtue of higher memory-subsystem performance. But it's not something you'd notice without benchmarking IMHO. The A7N266-VM has proven very stable in our fleet (running Win2000/Office2000 Pro) and I would recommend it as the better value due to its onboard NIC and video.
Two other options: look for a KM400 board (KT400 plus onboard video and probably onboard NIC) or hold out for nForce2 boards that have the integrated video (dual-monitor support with the onboard video for the EPoX 8RGA+ sounds cool).