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.
Razorback - Arch-6430U - i7-3687U - 16GB - Intel HD4000
Rocinante - Ubuntu-E5530 - i5-3230M - 16GB - Intel HD4000
Donnager - Arch-M4800 - i7-4800MQ - 32GB - Intel HD4600/Quadro K2100M
Vargr - Arch-Vargr - i7-4790K - 32GB - Intel HD4600/GeForce GTX 970