With LGA 775 builds higher frequency RAM does not add to performance in any way outside of synthetic benchmarks. If you were going to overclock beyond a 400Mhz FSB, then you might need 1000Mhz or 1066Mhz RAM.
Because the E7400 runs on a 266Mhz FSB, it's doubtful you would ever increase the FSB beyond 400. If this were an E8400 which runs at a 333 FSB, going past 400 would be fairly common so faster RAM might be in order.