AMD Athlon 64 3400+
This is the fastest rated Athlon 64 with New Castle-core available on S754. The S939-based New Castle CPU's where rated higher (up to 3800+) due to the faster HyperTransport bus and dual-channel memory capabilities.
S754 did offer faster CPU's: the 3700+, also at 2400MHz, but with Clawhammer featuring 1MB L2-cache.