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.
pa' ngaSwI' nuq vay' Data'nISbogh roD tu'lu' tlhIH'a'?! yIn jaj tera' na'ran wIb laH HInob Qub 'oH rue munISbogh 'Iv ghaH DaSov'a'? loD chay'pen tuqwIj meQ 'Iv jIH! TERA' NA'RAN WIB! chay'pen burns tuq vI'ogh 'ogh, chombuStIble tera' na'ran wIb!