Yes, with all three slots full you'll likely be cruising at 333. No problem with two total DIMMs at 400.
The issue boils down to sides. The Athlon64 can handle 4 sides of ram at DDR400. If you use dual sided DIMMs (very common in 512MB+), you can only do two sticks. If you're lucky enough to find single sided DIMMs, you can fill all three slots.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.