Out of interest, have you done speed tests with a simple crossover cable between your two AMD64 machines? You know, just to see what they're able to achieve without anything else causing problems.
Having just tested mine (going via some netgear gigabit switch), I also get about 26.5mB/sec (according ot DU Meter).
Weird thing is, when I start to copy a large file, it does a few seconds at 46mB/sec before going down to that ~26 speed.