Athlon 64 3400+ Clawhammer speed 2.2ghz, L2 cache 1024 KB.
SONAR Test 3 score @ 1.5 ms: 57-63%.

Athlon 64 3400+ Newcastle speed 2.4ghz, L2 512KB
SONAR Test 3 score @ 1.5ms: 69-78%.

(Lower is better)

At 11.6ms the performance is identical. The Newcastle might have an
advantage at
higher latencies.

The processor is the only difference in hardware and settings.

The Clawhammer runs a couple of degrees Celsius hotter than the Newcastle.

The larger L2 of the Clawhammer seems to be a big deal.

So if you are about to upgrade now and low latency power is a priority for you, get the Clawhammer while they are still available.

For more info on the SONAR 3 test and on how other processors are doing on it go here:

To make a long story short, dual opteron systems are king, followed by A64 1MB L2 models.
The only Intel that compares to the A64 is the 2.0 Centrino. P4's basically suck for the money.